/dev/disk0 vs /dev/mmc0

Hi all,

I am following barebox git closely and noticed a change recently: Device names for MMC (MCI) and USB mass-storage devices have changed to the generic "/dev/diskX". Earlier an MMC device was named "/dev/mmc0". Unfortunately this change breaks my /env/bin/init script and I don't know how to fix it. I relied on the existence of certain devices to distinguish between USB mass-storage device presence and/or SD-card presence. How can I do this with this new device naming convention?

I used to have these kind of checks in /env/bin/init:

# Mount MMC (first partition) if available
if [ -e "/dev/mmc0.0" ]; then

        mkdir /mmc
        mount /dev/mmc0.0 /mmc
        if [ -e "/mmc/uImage" ]; then
                boot_target="mmc"
        fi

fi

Booting like this is only allowed from MMC and not from USB, and now it seems impossible to distinguish between them anymore.

Btw, why was this changed in the first place?

Quoted 22 lines · David Jander

Hi all,

I am following barebox git closely and noticed a change recently: Device names for MMC (MCI) and USB mass-storage devices have changed to the generic "/dev/diskX". Earlier an MMC device was named "/dev/mmc0". Unfortunately this change breaks my /env/bin/init script and I don't know how to fix it. I relied on the existence of certain devices to distinguish between USB mass-storage device presence and/or SD-card presence. How can I do this with this new device naming convention?

I used to have these kind of checks in /env/bin/init:

# Mount MMC (first partition) if available
if [ -e "/dev/mmc0.0" ]; then

        mkdir /mmc
        mount /dev/mmc0.0 /mmc
        if [ -e "/mmc/uImage" ]; then
                boot_target="mmc"
        fi

fi

Quoted 4 lines

Booting like this is only allowed from MMC and not from USB, and now it seems impossible to distinguish between them anymore.

Btw, why was this changed in the first place?

now you an use devname parameter to specify a specifc name for mmc

not all the drivers have the platform_data update to support but it will be easy enough to add it

I recently add this to atmel_mci and animeo_ip board

and I recomment you to switch to defaultenv-2 for this

and use boot sequence this will simplify your env hugely

Hi Jean-Christophe,

Quoted 26 lines · On Thu, 3 Oct 2013 21:23:49 +0200 Jean-Christophe PLAGNIOL-VILLARD…

Quoted 25 lines · David Jander

Hi all,

I am following barebox git closely and noticed a change recently: Device names for MMC (MCI) and USB mass-storage devices have changed to the generic "/dev/diskX". Earlier an MMC device was named "/dev/mmc0". Unfortunately this change breaks my /env/bin/init script and I don't know how to fix it. I relied on the existence of certain devices to distinguish between USB mass-storage device presence and/or SD-card presence. How can I do this with this new device naming convention?

I used to have these kind of checks in /env/bin/init:

# Mount MMC (first partition) if available
if [ -e "/dev/mmc0.0" ]; then

        mkdir /mmc
        mount /dev/mmc0.0 /mmc
        if [ -e "/mmc/uImage" ]; then
                boot_target="mmc"
        fi

fi

Booting like this is only allowed from MMC and not from USB, and now it seems impossible to distinguish between them anymore.

Quoted 3 lines

Quoted 1 line

Btw, why was this changed in the first place?

now you an use devname parameter to specify a specifc name for mmc

My board is fully device-tree based, but your suggestion pointed me in the right direction: the "alias" section seems to have a whole new meaning now ;-) I hope this won't interfere with Linux, since I intend to use a single DT for both Barebox _and_ Linux.

Quoted 5 lines

not all the drivers have the platform_data update to support but it will be easy enough to add it

I recently add this to atmel_mci and animeo_ip board

Didn't find that one... probably not in mainline yet? Never mind.

Quoted 3 lines

and I recomment you to switch to defaultenv-2 for this

and use boot sequence this will simplify your env hugely

Although I agree defaultenv-2 looks pretty cool and will add a lot of convenience and flexibility for complex systems, IMHO it will not simplify the env of this board the least bit :-)
In fact, on a simple embedded system that has to have a strictly predefined boot-behavior with a few simple rules, it is overly complex and if an "outsider" ever needs to understand how it works, that can be a bit of a problem (the default boot path probably going through some 5 different scripts with lots of compound config-variables).

Quoted 32 lines · David Jander

Hi Jean-Christophe,

Quoted 26 lines · On Thu, 3 Oct 2013 21:23:49 +0200 Jean-Christophe PLAGNIOL-VILLARD…

Quoted 25 lines · David Jander

Hi all,

I am following barebox git closely and noticed a change recently: Device names for MMC (MCI) and USB mass-storage devices have changed to the generic "/dev/diskX". Earlier an MMC device was named "/dev/mmc0". Unfortunately this change breaks my /env/bin/init script and I don't know how to fix it. I relied on the existence of certain devices to distinguish between USB mass-storage device presence and/or SD-card presence. How can I do this with this new device naming convention?

I used to have these kind of checks in /env/bin/init:

# Mount MMC (first partition) if available
if [ -e "/dev/mmc0.0" ]; then

        mkdir /mmc
        mount /dev/mmc0.0 /mmc
        if [ -e "/mmc/uImage" ]; then
                boot_target="mmc"
        fi

fi

Booting like this is only allowed from MMC and not from USB, and now it seems impossible to distinguish between them anymore.

Quoted 8 lines

Quoted 3 lines

Quoted 1 line

Btw, why was this changed in the first place?

now you an use devname parameter to specify a specifc name for mmc

My board is fully device-tree based, but your suggestion pointed me in the right direction: the "alias" section seems to have a whole new meaning now ;-) I hope this won't interfere with Linux, since I intend to use a single DT for both Barebox _and_ Linux.

It doesn't interfere with the kernel. The kernel currently ignores this aliases. There are patches floating to let the kernel honor this aliases, but then they should simply have the same effect as they have in barebox.

Quoted 12 lines

Quoted 5 lines

not all the drivers have the platform_data update to support but it will be easy enough to add it

I recently add this to atmel_mci and animeo_ip board

Didn't find that one... probably not in mainline yet? Never mind.

Quoted 3 lines

and I recomment you to switch to defaultenv-2 for this

and use boot sequence this will simplify your env hugely

Quoted 8 lines

Although I agree defaultenv-2 looks pretty cool and will add a lot of convenience and flexibility for complex systems, IMHO it will not simplify the env of this board the least bit :-)
In fact, on a simple embedded system that has to have a strictly predefined boot-behavior with a few simple rules, it is overly complex and if an "outsider" ever needs to understand how it works, that can be a bit of a problem (the default boot path probably going through some 5 different scripts with lots of compound config-variables).

At least I recently replaced the boot script with a command (not yet mainline), so there are fewer scripts involved.

Sascha

Signature · 4 lines

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Dear Sascha,

Quoted 5 lines · On Sun, 6 Oct 2013 12:39:50 +0200 Sascha Hauer <[email protected]> wrote:

It doesn't interfere with the kernel. The kernel currently ignores this aliases. There are patches floating to let the kernel honor this aliases, but then they should simply have the same effect as they have in barebox.

That doesn't _feel_ right. Changing standard device names in Linux via aliases in the DT might be a fancy idea, but it'd have a different reason/use-case than in the case of barebox. IMHO using the same DT for both seems to be the Right Thing (tm) to do, but then the semantics must be the same also. If I need aliases in the DT only to be able to tell devices apart from each other in barebox, while in Linux the effect would only be a rather inconvenient renaming of devices with no other added value, I think we need a different way to differentiate devices in barebox. Why not just use a simple driver-provided prefix (mci, mmc, usb, sata, etc...) for now?

Quoted 11 lines · David Jander

Dear Sascha,

Quoted 5 lines · On Sun, 6 Oct 2013 12:39:50 +0200 Sascha Hauer <[email protected]> wrote:

It doesn't interfere with the kernel. The kernel currently ignores this aliases. There are patches floating to let the kernel honor this aliases, but then they should simply have the same effect as they have in barebox.

Quoted 10 lines

That doesn't _feel_ right. Changing standard device names in Linux via aliases in the DT might be a fancy idea, but it'd have a different reason/use-case than in the case of barebox. IMHO using the same DT for both seems to be the Right Thing (tm) to do, but then the semantics must be the same also. If I need aliases in the DT only to be able to tell devices apart from each other in barebox, while in Linux the effect would only be a rather inconvenient renaming of devices with no other added value, I think we need a different way to differentiate devices in barebox. Why not just use a simple driver-provided prefix (mci, mmc, usb, sata, etc...) for now?

That's not enough. We also need a fixed numbering. Otherwise a nonremovable eMMC and a removable SD card change their device names depending on the detect order.

Using aliases to provide a numbering is done in the Kernel aswell at least for gpios, uarts and i2c busses, so expanding this scheme to mmc/sd slots doesn't feel too wrong to me.

Quoted 23 lines · On Mon, 7 Oct 2013 08:41:11 +0200 Sascha Hauer <[email protected]> wrote:

Quoted 21 lines · David Jander

Dear Sascha,

Quoted 5 lines · On Sun, 6 Oct 2013 12:39:50 +0200 Sascha Hauer <[email protected]> wrote:

It doesn't interfere with the kernel. The kernel currently ignores this aliases. There are patches floating to let the kernel honor this aliases, but then they should simply have the same effect as they have in barebox.

That doesn't _feel_ right. Changing standard device names in Linux via aliases in the DT might be a fancy idea, but it'd have a different reason/use-case than in the case of barebox. IMHO using the same DT for both seems to be the Right Thing (tm) to do, but then the semantics must be the same also. If I need aliases in the DT only to be able to tell devices apart from each other in barebox, while in Linux the effect would only be a rather inconvenient renaming of devices with no other added value, I think we need a different way to differentiate devices in barebox. Why not just use a simple driver-provided prefix (mci, mmc, usb, sata, etc...) for now?

Quoted 3 lines

That's not enough. We also need a fixed numbering. Otherwise a nonremovable eMMC and a removable SD card change their device names depending on the detect order.

True. Can host->dev.id be used? Should be fixed AFAICS...

Quoted 4 lines

Using aliases to provide a numbering is done in the Kernel aswell at least for gpios, uarts and i2c busses, so expanding this scheme to mmc/sd slots doesn't feel too wrong to me.

Yes, but we are doing it in barebox now only because there is no other way to tell devices apart from each other. While the MMC device being called "mmcblk0" or whatever in Linux is perfectly fine (no need for alias),

in
barebox the device is now named "disk0", and there is no way of knowing what "disk0" actually is. Using DT-aliases for that purpose seems wrong to me. Why was this changed anyway? Introduction of some common "disk" layer (like scsi-disk on Linux)? Or just for the sake of confusing it with other "disk"-like devices?

Quoted 26 lines · David Jander

Quoted 23 lines · On Mon, 7 Oct 2013 08:41:11 +0200 Sascha Hauer <[email protected]> wrote:

Quoted 21 lines · David Jander

Dear Sascha,

Quoted 5 lines · On Sun, 6 Oct 2013 12:39:50 +0200 Sascha Hauer <[email protected]> wrote:

It doesn't interfere with the kernel. The kernel currently ignores this aliases. There are patches floating to let the kernel honor this aliases, but then they should simply have the same effect as they have in barebox.

That doesn't _feel_ right. Changing standard device names in Linux via aliases in the DT might be a fancy idea, but it'd have a different reason/use-case than in the case of barebox. IMHO using the same DT for both seems to be the Right Thing (tm) to do, but then the semantics must be the same also. If I need aliases in the DT only to be able to tell devices apart from each other in barebox, while in Linux the effect would only be a rather inconvenient renaming of devices with no other added value, I think we need a different way to differentiate devices in barebox. Why not just use a simple driver-provided prefix (mci, mmc, usb, sata, etc...) for now?

Quoted 5 lines

Quoted 3 lines

That's not enough. We also need a fixed numbering. Otherwise a nonremovable eMMC and a removable SD card change their device names depending on the detect order.

True. Can host->dev.id be used? Should be fixed AFAICS...

host->dev.id depends on the probe order.

Quoted 7 lines

Quoted 3 lines

Using aliases to provide a numbering is done in the Kernel aswell at least for gpios, uarts and i2c busses, so expanding this scheme to mmc/sd slots doesn't feel too wrong to me.

Yes, but we are doing it in barebox now only because there is no other way to tell devices apart from each other. While the MMC device being called "mmcblk0" or whatever in Linux is perfectly fine (no need for alias),

Linux has exactly the same problem. There are enough systems on which the eMMC changes its name depending on a SD card being plugged in or not.

Quoted 6 lines

in
barebox the device is now named "disk0", and there is no way of knowing what "disk0" actually is. Using DT-aliases for that purpose seems wrong to me. Why was this changed anyway? Introduction of some common "disk" layer (like scsi-disk on Linux)? Or just for the sake of confusing it with other "disk"-like devices?

It has always been diskx on barebox, this behaviour hasn't changed.

Quoted 27 lines · On Mon, 7 Oct 2013 22:19:36 +0200 Sascha Hauer <[email protected]> wrote:

Quoted 26 lines · David Jander

Quoted 23 lines · On Mon, 7 Oct 2013 08:41:11 +0200 Sascha Hauer <[email protected]> wrote:

Quoted 21 lines · David Jander

Dear Sascha,

Quoted 5 lines · On Sun, 6 Oct 2013 12:39:50 +0200 Sascha Hauer <[email protected]> wrote:

It doesn't interfere with the kernel. The kernel currently ignores this aliases. There are patches floating to let the kernel honor this aliases, but then they should simply have the same effect as they have in barebox.

That doesn't _feel_ right. Changing standard device names in Linux via aliases in the DT might be a fancy idea, but it'd have a different reason/use-case than in the case of barebox. IMHO using the same DT for both seems to be the Right Thing (tm) to do, but then the semantics must be the same also. If I need aliases in the DT only to be able to tell devices apart from each other in barebox, while in Linux the effect would only be a rather inconvenient renaming of devices with no other added value, I think we need a different way to differentiate devices in barebox. Why not just use a simple driver-provided prefix (mci, mmc, usb, sata, etc...) for now?

Quoted 7 lines

Quoted 5 lines

Quoted 3 lines

That's not enough. We also need a fixed numbering. Otherwise a nonremovable eMMC and a removable SD card change their device names depending on the detect order.

True. Can host->dev.id be used? Should be fixed AFAICS...

host->dev.id depends on the probe order.

:-(

Quoted 11 lines

Quoted 7 lines

Quoted 3 lines

Using aliases to provide a numbering is done in the Kernel aswell at least for gpios, uarts and i2c busses, so expanding this scheme to mmc/sd slots doesn't feel too wrong to me.

Yes, but we are doing it in barebox now only because there is no other way to tell devices apart from each other. While the MMC device being called "mmcblk0" or whatever in Linux is perfectly fine (no need for alias),

Linux has exactly the same problem. There are enough systems on which the eMMC changes its name depending on a SD card being plugged in or not.

Yes, but you have udev/mdev hotplugging and the sysfs interface to figure out what you need to know. Barebox has nothing like that AFAICS.

Quoted 8 lines

Quoted 6 lines

in
barebox the device is now named "disk0", and there is no way of knowing what "disk0" actually is. Using DT-aliases for that purpose seems wrong to me. Why was this changed anyway? Introduction of some common "disk" layer (like scsi-disk on Linux)? Or just for the sake of confusing it with other "disk"-like devices?

It has always been diskx on barebox, this behaviour hasn't changed.

You are right. Sorry for the confusion. There were aliases in barebox's version of imx53.dtsi which went missing somehow in my tree. I have been merging stuff from mainline Linux....
That leads me to the question: What is the intended relationship between barebox and Linux kernel DTS files? Which one is supposed to be the "master" or "upstream" version? Aren't they supposed to be kept in sync somehow? At least the basic SoC-specific .dtsi files?

Am Dienstag, den 08.10.2013, 09:02 +0200 schrieb David Jander:

Quoted 66 lines

Quoted 28 lines · On Mon, 7 Oct 2013 22:19:36 +0200 Sascha Hauer <[email protected]> wrote:

Quoted 27 lines · David Jander

Quoted 24 lines · On Mon, 7 Oct 2013 08:41:11 +0200 Sascha Hauer <[email protected]> wrote:

Quoted 22 lines · David Jander

Dear Sascha,

Quoted 5 lines · On Sun, 6 Oct 2013 12:39:50 +0200 Sascha Hauer <[email protected]> wrote:

It doesn't interfere with the kernel. The kernel currently ignores this aliases. There are patches floating to let the kernel honor this aliases, but then they should simply have the same effect as they have in barebox.

That doesn't _feel_ right. Changing standard device names in Linux via aliases in the DT might be a fancy idea, but it'd have a different reason/use-case than in the case of barebox. IMHO using the same DT for both seems to be the Right Thing (tm) to do, but then the semantics must be the same also. If I need aliases in the DT only to be able to tell devices apart from each other in barebox, while in Linux the effect would only be a rather inconvenient renaming of devices with no other added value, I think we need a different way to differentiate devices in barebox. Why not just use a simple driver-provided prefix (mci, mmc, usb, sata, etc...) for now?

Quoted 7 lines

Quoted 5 lines

Quoted 3 lines

That's not enough. We also need a fixed numbering. Otherwise a nonremovable eMMC and a removable SD card change their device names depending on the detect order.

True. Can host->dev.id be used? Should be fixed AFAICS...

host->dev.id depends on the probe order.

:-(

Quoted 4 lines

Quoted 4 lines

Quoted 3 lines

Using aliases to provide a numbering is done in the Kernel aswell at least for gpios, uarts and i2c busses, so expanding this scheme to mmc/sd slots doesn't feel too wrong to me.

Quoted 8 lines

Quoted 4 lines

Yes, but we are doing it in barebox now only because there is no other way to tell devices apart from each other. While the MMC device being called "mmcblk0" or whatever in Linux is perfectly fine (no need for alias),

Linux has exactly the same problem. There are enough systems on which the eMMC changes its name depending on a SD card being plugged in or not.

Yes, but you have udev/mdev hotplugging and the sysfs interface to figure out what you need to know. Barebox has nothing like that AFAICS.

Quoted 8 lines

Quoted 6 lines

in
barebox the device is now named "disk0", and there is no way of knowing what "disk0" actually is. Using DT-aliases for that purpose seems wrong to me. Why was this changed anyway? Introduction of some common "disk" layer (like scsi-disk on Linux)? Or just for the sake of confusing it with other "disk"-like devices?

It has always been diskx on barebox, this behaviour hasn't changed.

Quoted 8 lines

You are right. Sorry for the confusion. There were aliases in barebox's version of imx53.dtsi which went missing somehow in my tree. I have been merging stuff from mainline Linux....
That leads me to the question: What is the intended relationship between barebox and Linux kernel DTS files? Which one is supposed to be the "master" or "upstream" version? Aren't they supposed to be kept in sync somehow? At least the basic SoC-specific .dtsi files?

Missing other options we regard the Linux DTs as the upstream. Ideally DTs should be stored in a separate repo, so they don't get broken unnecessarily.

We will try to keep the Barebox DTs in sync with Linux as much as possible. But even then there is no guarantee that Barebox and Linux DTs don't drift away from each other over time.
If a binding gets broken (let's hope that everyone gets their act together and don't do this anymore), it will require code changes and maintenance work to get things in sync again.

Another real-life use-case where you might end up with different DTs is if you update your Linux kernel more regularly than the Barebox on your device. So please if ever possible don't boot the Linux kernel with the builtin Barebox DT, but design your boot concept in a way that you always deliver a DTB fitting your kernel.

Quoted 19 lines

Quoted 8 lines

You are right. Sorry for the confusion. There were aliases in barebox's version of imx53.dtsi which went missing somehow in my tree. I have been merging stuff from mainline Linux....
That leads me to the question: What is the intended relationship between barebox and Linux kernel DTS files? Which one is supposed to be the "master" or "upstream" version? Aren't they supposed to be kept in sync somehow? At least the basic SoC-specific .dtsi files?

Missing other options we regard the Linux DTs as the upstream. Ideally DTs should be stored in a separate repo, so they don't get broken unnecessarily.

We will try to keep the Barebox DTs in sync with Linux as much as possible. But even then there is no guarantee that Barebox and Linux DTs don't drift away from each other over time.
If a binding gets broken (let's hope that everyone gets their act together and don't do this anymore), it will require code changes and maintenance work to get things in sync again.

Quoted 5 lines

Another real-life use-case where you might end up with different DTs is if you update your Linux kernel more regularly than the Barebox on your device. So please if ever possible don't boot the Linux kernel with the builtin Barebox DT, but design your boot concept in a way that you always deliver a DTB fitting your kernel.

Our boot-scheme checks for a DTB on the boot-medium. If it finds one matching the board, it is used, otherwise it boots with barebox's built-in DTB, and that should be the default situation, unless it becomes necessary to overrule the internal DTB for whatever regrettable reason.

I have had strong discussions (almost flame-war) long time ago on alkml about the issue of hardware initialization. IMNSHO, hardware initialization that has to do with the electrical layout of the board (PAD settings, drive-strength, etc...) must always be done in the boot-loader. Back in the time where there was no DT for ARM, Linux kernel board support code was a bloody mess with this sort of HW initialization everywhere, and everywhere done differently. Only the hardware designer know what drive-strength and other PAD settings need to

be chosen for each PAD. That is where your design is based on. Kernel (-developers) do not know, and should not need to care.
Now there is this "invention" of putting all this information in the DT. This actually seems like a good compromise between what I said above and the fact that the kernel sometimes needs to reconfigure PADs due to PM, pluggable devices and such. But still the knowledge about PAD settings (and what hardware the board happens to have for that matter) belongs to the boot-loader. Having more than one source for this is dangerous.

Am Dienstag, den 08.10.2013, 11:13 +0200 schrieb David Jander:
[...]

Quoted 25 lines

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Quoted 8 lines

You are right. Sorry for the confusion. There were aliases in barebox's version of imx53.dtsi which went missing somehow in my tree. I have been merging stuff from mainline Linux....
That leads me to the question: What is the intended relationship between barebox and Linux kernel DTS files? Which one is supposed to be the "master" or "upstream" version? Aren't they supposed to be kept in sync somehow? At least the basic SoC-specific .dtsi files?

Missing other options we regard the Linux DTs as the upstream. Ideally DTs should be stored in a separate repo, so they don't get broken unnecessarily.

We will try to keep the Barebox DTs in sync with Linux as much as possible. But even then there is no guarantee that Barebox and Linux DTs don't drift away from each other over time.
If a binding gets broken (let's hope that everyone gets their act together and don't do this anymore), it will require code changes and maintenance work to get things in sync again.

Another real-life use-case where you might end up with different DTs is if you update your Linux kernel more regularly than the Barebox on your device. So please if ever possible don't boot the Linux kernel with the builtin Barebox DT, but design your boot concept in a way that you always deliver a DTB fitting your kernel.

Quoted 5 lines

Our boot-scheme checks for a DTB on the boot-medium. If it finds one matching the board, it is used, otherwise it boots with barebox's built-in DTB, and that should be the default situation, unless it becomes necessary to overrule the internal DTB for whatever regrettable reason.

Yeah that's the conclusion you may be inclined to take if you try to live in an ideal world. (Including a dot-shaped earth in a vacuum.)

Experience with the device tree shows that there are a lot more of those regrettable reasons than one might imagine. So in order to get around all those real world problems always boot your kernel with a devicetree matching exactly that kernel version. Take this as a well-meaning advise from someone who hit the hard wall of the real-world device tree more than one.

Quoted 8 lines

I have had strong discussions (almost flame-war) long time ago on alkml about the issue of hardware initialization. IMNSHO, hardware initialization that has to do with the electrical layout of the board (PAD settings, drive-strength, etc...) must always be done in the boot-loader. Back in the time where there was no DT for ARM, Linux kernel board support code was a bloody mess with this sort of HW initialization everywhere, and everywhere done differently. Only the hardware designer know what drive-strength and other PAD settings need to be chosen for each PAD. That

Quoted 9 lines

is where your design is based on. Kernel (-developers) do not know, and should not need to care. Now there is this "invention" of putting all this information in the DT. This actually seems like a good compromise between what I said above and the fact that the kernel sometimes needs to reconfigure PADs due to PM, pluggable devices and such. But still the knowledge about PAD settings (and what hardware the board happens to have for that matter) belongs to the boot-loader. Having more than one source for this is dangerous.

No, that is utterly wrong. Linux should always know everything about your hardware and _not_ expect anything to be set up by the bootloader.

There are some corner cases like chip errata fixes here, but generally don't assume the bootloader to set things up for you.

The bootloader
should only do the minimal needed init for loading the kernel and booting. The kernel is your hardware abstraction for your running system, not the bootloader.

Ideally both the bootloader and kernel would source their needed information from a shared DT that's decoupled from kernel development and stable, once again reality strikes in here. I agree with you that not a kernel developer, but board designer should write the DT and we might actually get this separation in a clean way, if we manage to split out the DTs from the kernel some day.

Dear Lucas,

Quoted 27 lines · On Tue, 08 Oct 2013 11:39:24 +0200 Lucas Stach <[email protected]> wrote:

Am Dienstag, den 08.10.2013, 11:13 +0200 schrieb David Jander:
[...]

Quoted 25 lines

Quoted 24 lines

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You are right. Sorry for the confusion. There were aliases in barebox's version of imx53.dtsi which went missing somehow in my tree. I have been merging stuff from mainline Linux....
That leads me to the question: What is the intended relationship between barebox and Linux kernel DTS files? Which one is supposed to be the "master" or "upstream" version? Aren't they supposed to be kept in sync somehow? At least the basic SoC-specific .dtsi files?

Missing other options we regard the Linux DTs as the upstream. Ideally DTs should be stored in a separate repo, so they don't get broken unnecessarily.

We will try to keep the Barebox DTs in sync with Linux as much as possible. But even then there is no guarantee that Barebox and Linux DTs don't drift away from each other over time.
If a binding gets broken (let's hope that everyone gets their act together and don't do this anymore), it will require code changes and maintenance work to get things in sync again.

Another real-life use-case where you might end up with different DTs is if you update your Linux kernel more regularly than the Barebox on your device. So please if ever possible don't boot the Linux kernel with the builtin Barebox DT, but design your boot concept in a way that you always deliver a DTB fitting your kernel.

Quoted 7 lines

Quoted 5 lines

Our boot-scheme checks for a DTB on the boot-medium. If it finds one matching the board, it is used, otherwise it boots with barebox's built-in DTB, and that should be the default situation, unless it becomes necessary to overrule the internal DTB for whatever regrettable reason.

Yeah that's the conclusion you may be inclined to take if you try to live in an ideal world. (Including a dot-shaped earth in a vacuum.)

Aren't we living on a dot-shaped earth in a vacuum?

Quoted 6 lines

Experience with the device tree shows that there are a lot more of those regrettable reasons than one might imagine. So in order to get around all those real world problems always boot your kernel with a devicetree matching exactly that kernel version. Take this as a well-meaning advise from someone who hit the hard wall of the real-world device tree more than one.

Don't get me wrong, I have had enough cases of device-trees changing and kernels being incompatible with them. I have hit the hard wall enough times already, but that doesn't mean it hasn't always been for pretty regrettable reasons. Open Firmware and Device-Trees are both good ideas and we are not using them (yet) as they are supposed to be used. Saying that we shouldn't just because we haven't been able to make it work as intended sounds more like an excuse to me than a hard fact.... ;-)
Make the problem be the exception, not the rule.

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I have had strong discussions (almost flame-war) long time ago on alkml about the issue of hardware initialization. IMNSHO, hardware initialization that has to do with the electrical layout of the board (PAD settings, drive-strength, etc...) must always be done in the boot-loader. Back in the time where there was no DT for ARM, Linux kernel board support code was a bloody mess with this sort of HW initialization everywhere, and everywhere done differently. Only the hardware designer know what drive-strength and other PAD settings need to be chosen for each PAD. That is where your design is based on. Kernel (-developers) do not know, and should not need to care. Now there is this "invention" of putting all this information in the DT. This actually seems like a good compromise between what I said above and the fact that the kernel sometimes needs to reconfigure PADs due to PM, pluggable devices and such. But still the knowledge about PAD settings (and what hardware the board happens to have for that matter) belongs to the boot-loader. Having more than one source for this is dangerous.

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No, that is utterly wrong. Linux should always know everything about your hardware and _not_ expect anything to be set up by the bootloader.

Huh? AFAIK, it has never been like that. PC's have BIOS (or UEFI firmware for that matter) that takes care of signal integrity setup of the PC's chipset, motherboard, CPU and RAM. Next, do you want to leave DDR timing setup to the Linux kernel?? You are kidding, right?

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There are some corner cases like chip errata fixes here, but generally don't assume the bootloader to set things up for you.

Another case of making the problem the rule instead of the exception.... The fact that board manufacturers don't get their act together to make decent firmware (bootloader) is no excuse for saying it shouldn't be done like that.

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The bootloader
should only do the minimal needed init for loading the kernel and booting. The kernel is your hardware abstraction for your running system, not the bootloader.

That is ok. What I am saying is that settings that have strictly to do with _hardware_ constraints should never be bothered by the Linux kernel. Otherwise you would need to recompile the kernel (or change the device-tree) for potentially every single hardware revision of one board model.
If I place other RAM chips on the board for instance, or if there is a layout revision, or a peripheral chip that has had a die revision requiring different drive-strength and such changes.... that information should be taken care of by the boot-loader.

Configuring peripherals, setting MUX registers and such is a different story. That should (and many times must) be done in the kernel.

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Ideally both the bootloader and kernel would source their needed information from a shared DT that's decoupled from kernel development and stable, once again reality strikes in here. I agree with you that not a kernel developer, but board designer should write the DT and we might actually get this separation in a clean way, if we manage to split out the DTs from the kernel some day.

Well, I am a board designer, and I am trying to do just that. Pardon me for being an idealist sometimes ;-)

Funny enough, some of my gripes about the state of ARM-Linux initially were echoed years later by Linus himself in his famous e-mail about the mess that was ARM-Linux board support before DT came along. Back then, when I asked why ARM wouldn't benefit from OF-device-trees just like PowerPC and Sparc did, I was being laughed at. Unfortunately a few conceptual things have been implemented the wrong way (like PAD setup), but hey! Not bad ;-)

Am Dienstag, den 08.10.2013, 15:47 +0200 schrieb David Jander:

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Dear Lucas,

Quoted 28 lines · On Tue, 08 Oct 2013 11:39:24 +0200 Lucas Stach <[email protected]> wrote:

Am Dienstag, den 08.10.2013, 11:13 +0200 schrieb David Jander:
[...]

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You are right. Sorry for the confusion. There were aliases in barebox's version of imx53.dtsi which went missing somehow in my tree. I have been merging stuff from mainline Linux.... That leads me to the question: What is the intended relationship between barebox and Linux kernel DTS files? Which one is supposed to be the "master" or "upstream" version? Aren't they supposed to be kept in sync somehow? At least the basic SoC-specific .dtsi files?

Missing other options we regard the Linux DTs as the upstream. Ideally DTs should be stored in a separate repo, so they don't get broken unnecessarily.

We will try to keep the Barebox DTs in sync with Linux as much as possible. But even then there is no guarantee that Barebox and Linux DTs don't drift away from each other over time.
If a binding gets broken (let's hope that everyone gets their act together and don't do this anymore), it will require code changes and maintenance work to get things in sync again.

Another real-life use-case where you might end up with different DTs is if you update your Linux kernel more regularly than the Barebox on your device. So please if ever possible don't boot the Linux kernel with the builtin Barebox DT, but design your boot concept in a way that you always deliver a DTB fitting your kernel.

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Our boot-scheme checks for a DTB on the boot-medium. If it finds one matching the board, it is used, otherwise it boots with barebox's built-in DTB, and that should be the default situation, unless it becomes necessary to overrule the internal DTB for whatever regrettable reason.

Yeah that's the conclusion you may be inclined to take if you try to live in an ideal world. (Including a dot-shaped earth in a vacuum.)

Aren't we living on a dot-shaped earth in a vacuum?

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Experience with the device tree shows that there are a lot more of those regrettable reasons than one might imagine. So in order to get around all those real world problems always boot your kernel with a devicetree matching exactly that kernel version. Take this as a well-meaning advise from someone who hit the hard wall of the real-world device tree more than one.

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Don't get me wrong, I have had enough cases of device-trees changing and kernels being incompatible with them. I have hit the hard wall enough times already, but that doesn't mean it hasn't always been for pretty regrettable reasons. Open Firmware and Device-Trees are both good ideas and we are not using them (yet) as they are supposed to be used. Saying that we shouldn't just because we haven't been able to make it work as intended sounds more like an excuse to me than a hard fact.... ;-)
Make the problem be the exception, not the rule.

Without doubt we all should always treat DTs the way we would like them to be used in the ideal case, but don't depend on those assumptions for your real products.

Using the builtin DTB normally, but having a way to override it when needed sounds like a reasonable compromise to make.

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I have had strong discussions (almost flame-war) long time ago on alkml about the issue of hardware initialization. IMNSHO, hardware initialization that has to do with the electrical layout of the board (PAD settings, drive-strength, etc...) must always be done in the boot-loader. Back in the time where there was no DT for ARM, Linux kernel board support code was a bloody mess with this sort of HW initialization everywhere, and everywhere done differently. Only the hardware designer know what drive-strength and other PAD settings need to be chosen for each PAD. That is where your design is based on.

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Kernel (-developers) do not know, and should not need to care. Now there is this "invention" of putting all this information in the DT. This actually seems like a good compromise between what I said above and the fact that the kernel sometimes needs to reconfigure PADs due to PM, pluggable devices and such. But still the knowledge about PAD settings (and what hardware the board happens to have for that matter) belongs to the boot-loader. Having more than one source for this is dangerous.

No, that is utterly wrong. Linux should always know everything about your hardware and _not_ expect anything to be set up by the bootloader.

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Huh? AFAIK, it has never been like that. PC's have BIOS (or UEFI firmware for that matter) that takes care of signal integrity setup of the PC's chipset, motherboard, CPU and RAM. Next, do you want to leave DDR timing setup to the Linux kernel?? You are kidding, right?

And those BIOS settings are so regularly wrong that the kernel has to ship a lot of cludge just to work around or correct that bootloader setup.

I agree that the bootloader should set up things needed for the boot, but in many cases the kernel needs the same knowledge anyway. Your DDR timing is a pretty prime example for this. The bootloader has to set it up initially, but the Linux kernel has to change timings dynamically (including calibration in some situations) when doing dynamic memory clock scaling.

CPU bringup is the same matter, Linux needs to know how to do it anyway as we need it for CPU hotplug when saving power.

Just because there is a real small amount of static setup that could be done exclusively in the bootloader, doesn't mean we should always force it down into the bootloader and tell Linux to depend on it.

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There are some corner cases like chip errata fixes here, but generally don't assume the bootloader to set things up for you.

Another case of making the problem the rule instead of the exception.... The fact that board manufacturers don't get their act together to make decent firmware (bootloader) is no excuse for saying it shouldn't be done like that.

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The bootloader
should only do the minimal needed init for loading the kernel and booting. The kernel is your hardware abstraction for your running system, not the bootloader.

That is ok. What I am saying is that settings that have strictly to do with _hardware_ constraints should never be bothered by the Linux kernel. Otherwise you would need to recompile the kernel (or change the device-tree) for potentially every single hardware revision of one board model. If I place other RAM chips on the board for instance, or if there is a layout revision, or a peripheral chip that has had a die revision requiring different drive-strength and such changes.... that information should be taken care of by the boot-loader.

Configuring peripherals, setting MUX registers and such is a different story. That should (and many times must) be done in the kernel.

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Ideally both the bootloader and kernel would source their needed information from a shared DT that's decoupled from kernel development and stable, once again reality strikes in here. I agree with you that not a kernel developer, but board designer should write the DT and we might actually get this separation in a clean way, if we manage to split out the DTs from the kernel some day.

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Well, I am a board designer, and I am trying to do just that. Pardon me for being an idealist sometimes ;-)

We can all do our share to better the overall situation by being idealist, but avoid basing your decision for real-world products on assumptions that are only true for an ideal world. :)

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Funny enough, some of my gripes about the state of ARM-Linux initially were echoed years later by Linus himself in his famous e-mail about the mess that was ARM-Linux board support before DT came along. Back then, when I asked why ARM wouldn't benefit from OF-device-trees just like PowerPC and Sparc did, I was being laughed at. Unfortunately a few conceptual things have been implemented the wrong way (like PAD setup), but hey! Not bad ;-)

For DRAM clock scaling, only the clock divider or PLL should ever be changed. Any other parameters should stay the same. If this has really been done, I'd like to see it...
DRAM calibration settings have to do with signal propagation on the PCB. That has nothing to do with clock speeds or -settings.

Ok, now can we both please go back to work?