boot/uboot: add support for custom DT name

From: Konstantin Porotchkin <[email protected]>

Some u-boot default configuration files could be shared between targets and used for building images for multiple board types. The only difference between such builds is the DTB embedded in in the boot image for each specific platform.
This approach is widely used by Marvell, having the same u-boot configuration file for the entire SoC family, but allowing builds of multiple target flavors by supplying the device tree name through make command parameter DEVICE_TREE=xxx
This patch adds such capability to uboot module of the buildroot. The custome DT name could be defined by BR2_TARGET_UBOOT_CUSTOM_DTS_NAME entry.

Change-Id I69e193339b0369a736bdf98491b9914d24a54e17

Signed-off-by Konstantin Porotchkin <[email protected]>

-----Original Message-----
From: buildroot <[email protected]> On Behalf Of Thomas Petazzoni
Sent: Monday, December 24, 2018 15:48
To: Kostya Porotchkin <[email protected]>
Cc: Clemens Gruber <[email protected]>;
buildroot at buildroot.org
Subject: [EXT] Re: [Buildroot] [PATCH] boot/uboot: add support for custom DT name

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Hello Kostya,

+Clemens Gruber in Cc, who posted a related U-Boot patch.

Quoted 14 lines · kostap at marvell.com

From: Konstantin Porotchkin <[email protected]>

Some u-boot default configuration files could be shared between targets and used for building images for multiple board types. The only difference between such builds is the DTB embedded in in the boot image for each specific platform.
This approach is widely used by Marvell, having the same u-boot configuration file for the entire SoC family, but allowing builds of multiple target flavors by supplying the device tree name through make command parameter DEVICE_TREE=xxx This patch adds such capability to uboot module of the buildroot.
The custome DT name could be defined by BR2_TARGET_UBOOT_CUSTOM_DTS_NAME entry.

Quoted 2 lines

Change-Id I69e193339b0369a736bdf98491b9914d24a54e17

Signed-off-by Konstantin Porotchkin <[email protected]>

Reading this again, in fact it is very similar to patch
http://patchwork.ozlabs.org/patch/881197/ we already have in the queue. With a very important distinction: your patch passes DEVICE_TREE=, while the existing pending patch passes EXT_DTB=. The obvious question that comes up is: which one is right ? Or are these two orthogonal things ?

Thomas
--
Thomas Petazzoni, CTO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
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Hello Kostya,

+Clemens Gruber in Cc, who posted a related U-Boot patch.

Quoted 14 lines · kostap at marvell.com

From: Konstantin Porotchkin <[email protected]>

Some u-boot default configuration files could be shared between targets and used for building images for multiple board types. The only difference between such builds is the DTB embedded in in the boot image for each specific platform.
This approach is widely used by Marvell, having the same u-boot configuration file for the entire SoC family, but allowing builds of multiple target flavors by supplying the device tree name through make command parameter DEVICE_TREE=xxx
This patch adds such capability to uboot module of the buildroot. The custome DT name could be defined by BR2_TARGET_UBOOT_CUSTOM_DTS_NAME entry.

Quoted 2 lines

Change-Id I69e193339b0369a736bdf98491b9914d24a54e17

Signed-off-by Konstantin Porotchkin <[email protected]>

Reading this again, in fact it is very similar to patch
http://patchwork.ozlabs.org/patch/881197/ we already have in the queue. With a very important distinction: your patch passes DEVICE_TREE=, while the existing pending patch passes EXT_DTB=. The obvious question that comes up is: which one is right ? Or are these two orthogonal things ?

They solve different problems.

The reason for the EXT_DTB patch was to allow passing pre-built dtbs with metadata used for verified boot in U-Boot.
(It's a simple device tree blob which stores the public key for the FIT image with the kernel, initrd etc. and is generated by U-Boot mkimage)

Matthew Weber suggested alternatives like appending this blob to uboot.bin or decompiling the dtb and including the resulting dts via BR2_TARGET_UBOOT_CUSTOM_DTS_PATH.

Afaik, DEVICE_TREE is for user-specified "normal" DTs, e.g. for configuration sharing / to override CONFIG_DEFAULT_DEVICE_TREE. EXT_DTB is used for these extra DT blobs in verified boot scenarios.

Hello Clemens, Kostya,

Quoted 7 lines · On Mon, 24 Dec 2018 17:59:31 +0100 Clemens Gruber…

They solve different problems.

The reason for the EXT_DTB patch was to allow passing pre-built dtbs with metadata used for verified boot in U-Boot.
(It's a simple device tree blob which stores the public key for the FIT image with the kernel, initrd etc. and is generated by U-Boot mkimage)

Quoted 7 lines

Matthew Weber suggested alternatives like appending this blob to uboot.bin or decompiling the dtb and including the resulting dts via BR2_TARGET_UBOOT_CUSTOM_DTS_PATH.

Afaik, DEVICE_TREE is for user-specified "normal" DTs, e.g. for configuration sharing / to override CONFIG_DEFAULT_DEVICE_TREE. EXT_DTB is used for these extra DT blobs in verified boot scenarios.

I thought a bit about this. U-Boot in fact has tons of environment variables to tweak stuff. Instead of adding explicit options for each of them, perhaps we should simply add an option that allows to pass arbitrary environment variables to the U-Boot build. Something long the lines of:

config BR2_TARGET_UBOOT_CUSTOM_ENVVARS

	string "additional environment variables"
	help
	  ...

UBOOT_MAKE_OPTS += $(call qstrip,$(BR2_TARGET_UBOOT_CUSTOM_ENVVARS))

Not only this would allow to support more than EXT_DTB and DEVICE_TREE, but other variables. In addition, what worries me about EXT_DTB and DEVICE_TREE as individual options is that the semantic is hard to explain. Both are device trees, but they don't serve the same purpose.

Thoughts ?