clocksource: exynos_mct: allow mct to use 64-bit counter from coprocessor
cover clocksource: exynos_mct: allow mct to use 64-bit counter from coprocessor
Hi all,
year(s) ago it was discovered that MCT timer and ARM architectured timer
are the same hardware with different interface. Here [1].
I followed mail-list discussions about removing MCT and using arch timer for Exynos5-based SoCs but things aren't moving at least latest upstream kernel on odroid-xu3 will use MCT as default timers. Maybe the reason are some power-management related things that very specific to Samsung. I don't know.
Idea of this draft patchset comes from Doug patches when he tried to optimize read of 64-bit counter located in mmio. [2]
Why not using cp15 counter instead if possible?
Previous numbers for 1000000 gettimeofday() calls from userspace are about 1 ms. With this patches we have 0.5 ms or even better. So twice as fast.
Just as matter of interest i tried to perform 2000000 sched_yield() calls from userspace. I see around 20% speedup with patches applied.
I tried to use hackbench but it's hard to feel the difference, maybe speedup is 0.5% but with bad fluctuation.
Everything is tested on odroid-xu3.
Looks like Cortex-A9 based Samsung SoCs (odroid-u2, odroid-x) don't have arch timer.
[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2014
-May/256943.html
[2] https://lkml.org/lkml/2014/6/20/431
Current code created with such assumptions:
-- don't want to insert huge refactoring in MCT code;
-- target platform only ARMv7 Exynos5-based SoC that works in 32-bit mode so i don't want to build described functionality on ARM64. Currently i'm trying to patch odroid-xu3 DT only.
-- firmware on odroid-xu3 is broken and secondary cores start in SVC mode instead of HYP mode. Maybe i can remove check for hyp mode; -- in addition instead of DT property I may parse PFR regs and find Generic Timer Extension there.
I hope you are not getting bored reading to me. Current code is a little bit draft so comments and ideas are welcome.
Best regards,
Alexey Klimov
Quoted 5 lines · Alexey Klimov
Hi all,
year(s) ago it was discovered that MCT timer and ARM architectured timer
are the same hardware with different interface. Here [1].
Are they actually interfaces to the same timer, or are they just fed by the same system counter?
Quoted 10 lines
I followed mail-list discussions about removing MCT and using arch timer for Exynos5-based SoCs but things aren't moving at least latest upstream kernel on odroid-xu3 will use MCT as default timers. Maybe the reason are some power-management related things that very specific to Samsung. I don't know.
Idea of this draft patchset comes from Doug patches when he tried to optimize read of 64-bit counter located in mmio. [2]
Why not using cp15 counter instead if possible?
If they're the same device, then you can simply use the architected timer, and gain all the benefits without additional code. That could give you additional benefits (e.g. VDSO support).
Quoted 20 lines
Previous numbers for 1000000
gettimeofday()calls from userspace are about 1 ms. With this patches we have 0.5 ms or even better. So twice as fast.Just as matter of interest i tried to perform 2000000
sched_yield()calls from userspace. I see around 20% speedup with patches applied.I tried to use hackbench but it's hard to feel the difference, maybe speedup is 0.5% but with bad fluctuation.
Everything is tested on odroid-xu3.
Looks like Cortex-A9 based Samsung SoCs (odroid-u2, odroid-x) don't have arch timer.[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2014
-May/256943.html
[2] https://lkml.org/lkml/2014/6/20/431
Quoted 4 lines
Current code created with such assumptions:
-- don't want to insert huge refactoring in MCT code;
-- target platform only ARMv7 Exynos5-based SoC that works in 32-bit mode so i don't want to build described functionality on ARM64.
For arm64 the generic timer is mandatory, and I can't imagine a case where we wouldn't use it in preference.
Quoted 5 lines
Currently i'm trying to patch odroid-xu3 DT only.
-- firmware on odroid-xu3 is broken and secondary cores start in SVC mode instead of HYP mode. Maybe i can remove check for hyp mode; -- in addition instead of DT property I may parse PFR regs and find Generic Timer Extension there.
ID_PFR1.GenTimer only tells you whether or not the generic timer is implemented in the CPU, not how it's wired up (e.g. whether it shares the system counter with the MCT), so I don't think it tells you anything useful.
Thanks,
Mark.
Quoted 9 lines · Mark Rutland
Quoted 5 lines · Alexey Klimov
Hi all,
year(s) ago it was discovered that MCT timer and ARM architectured timer
are the same hardware with different interface. Here [1].Are they actually interfaces to the same timer, or are they just fed by the same system counter?
Good question goes to Samsung maintainers.
I can find unanswered mail thread:
http://lists.infradead.org/pipermail/linux-arm-kernel/2014-May/258576.html
Let me quote Kukjin: " Basically the two blocks are connected and the arch timer uses the count value from MCT for reference"
Quoted 7 lines
Quoted 7 lines
I followed mail-list discussions about removing MCT and using arch timer for Exynos5-based SoCs but things aren't moving at least latest upstream kernel on odroid-xu3 will use MCT as default timers. Maybe the reason are some power-management related things that very specific to Samsung. I don't know.
Quoted 7 lines
Quoted 3 lines
Idea of this draft patchset comes from Doug patches when he tried to optimize read of 64-bit counter located in mmio. [2]
Why not using cp15 counter instead if possible?If they're the same device, then you can simply use the architected timer, and gain all the benefits without additional code. That could give you additional benefits (e.g. VDSO support).
Right. I like the idea to use arch_timer. Odroid-xu3 works fine with it. Looks like Samsung have some games with power management stuff that can't be opened/told and they use MCT that works well for their needs. This draft patchset is only about little speedup of MCT operations that still here and used.
To start arch timer we may need to initialize G_TCON register in MCT like it's done in Xen:
http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/arch/arm/platforms/exynos5.c;h=79e3a5faf9fa6f37d99040f85c6dccbd9312c856;hb=HEAD
in exynos5_init_time(). I guess not all bootloaders properly initialize MCT on Exynos5 boards.
If addition of similar platform callback and enable of arch_timer are steps in right direction I can take a look on it.
Or as suggested in one mail thread: if MCT is present in DT then MCT driver lookup arch_timer in dt, find it, initialize G_TCON reg and gives up to allow arch_timer to do it job (in that case we need to init MCT before arch timer).
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Previous numbers for 1000000
gettimeofday()calls from userspace are about 1 ms. With this patches we have 0.5 ms or even better. So twice as fast.Just as matter of interest i tried to perform 2000000
sched_yield()calls from userspace. I see around 20% speedup with patches applied.I tried to use hackbench but it's hard to feel the difference, maybe speedup is 0.5% but with bad fluctuation.
Everything is tested on odroid-xu3.
Looks like Cortex-A9 based Samsung SoCs (odroid-u2, odroid-x) don't have arch timer.[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2014
-May/256943.html
[2] https://lkml.org/lkml/2014/6/20/431
Quoted 7 lines
Quoted 4 lines
Current code created with such assumptions:
-- don't want to insert huge refactoring in MCT code;
-- target platform only ARMv7 Exynos5-based SoC that works in 32-bit mode so i don't want to build described functionality on ARM64.For arm64 the generic timer is mandatory, and I can't imagine a case where we wouldn't use it in preference.
Yep. That's why probably MCT isn't used on arm64.
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Quoted 5 lines
Currently i'm trying to patch odroid-xu3 DT only.
-- firmware on odroid-xu3 is broken and secondary cores start in SVC mode instead of HYP mode. Maybe i can remove check for hyp mode; -- in addition instead of DT property I may parse PFR regs and find Generic Timer Extension there.
Quoted 7 lines
ID_PFR1.GenTimer only tells you whether or not the generic timer is implemented in the CPU, not how it's wired up (e.g. whether it shares the system counter with the MCT), so I don't think it tells you anything useful.
Thanks,
Mark.
Yep, just only one of the initial points to understand is it any use trying. Thanks.
Best regards,
Klimov Alexey
Alexey,
Quoted 13 lines · Alexey Klimov
Hi all,
year(s) ago it was discovered that MCT timer and ARM architectured timer
are the same hardware with different interface. Here [1].I followed mail-list discussions about removing MCT and using arch timer for Exynos5-based SoCs but things aren't moving at least latest upstream kernel on odroid-xu3 will use MCT as default timers. Maybe the reason are some power-management related things that very specific to Samsung. I don't know.
Quoted 3 lines
Idea of this draft patchset comes from Doug patches when he tried to optimize read of 64-bit counter located in mmio. [2]
Why not using cp15 counter instead if possible?
I hate to burst your bubble here, but...
...I think it would be a bad idea to use the cp15 counter on exynos 5422. According to Samsung, there are issues where using cp15 could sometimes return the wrong value, especially if you happen to read it while on an A7 instead of an A15. It will tend to work pretty well, but Samsung claimed that it might not work right on some CPUs or at some temperatures.
I've added Alim who provided this information to me (in the private
http://crosbug.com/p/29556). He may be able to elaborate more.
Generally I'd say that if the arch timer works for you: use the arch timer. If the arch timer doesn't work for you, use the MCT.
-Doug
Hi Doug,
Quoted 20 lines · Doug Anderson
Alexey,
Quoted 13 lines · Alexey Klimov
Hi all,
year(s) ago it was discovered that MCT timer and ARM architectured timer
are the same hardware with different interface. Here [1].I followed mail-list discussions about removing MCT and using arch timer for Exynos5-based SoCs but things aren't moving at least latest upstream kernel on odroid-xu3 will use MCT as default timers. Maybe the reason are some power-management related things that very specific to Samsung. I don't know.
Quoted 3 lines
Idea of this draft patchset comes from Doug patches when he tried to optimize read of 64-bit counter located in mmio. [2]
Why not using cp15 counter instead if possible?
Quoted 8 lines
I hate to burst your bubble here, but...
...I think it would be a bad idea to use the cp15 counter on exynos 5422. According to Samsung, there are issues where using cp15 could sometimes return the wrong value, especially if you happen to read it while on an A7 instead of an A15. It will tend to work pretty well, but Samsung claimed that it might not work right on some CPUs or at some temperatures.
I think this is right time when I can say "If only I had known about it before!".
That's quite interesting especially about temperature issues. Maybe it's worth to insert comments in driver/docs about this.
Quoted 2 lines
I've added Alim who provided this information to me (in the private
http://crosbug.com/p/29556). He may be able to elaborate more.
Alim, could you please add details if any?
Quoted 4 lines
Generally I'd say that if the arch timer works for you: use the arch timer. If the arch timer doesn't work for you, use the MCT.
-Doug
I possess no knowledge if there're any different hw revisions of odroid-xu3 on the field
but looks like arch timer works fine for me on odroid-xu3 Exynos5422 SoC with upstream kernel.
Is it right direction to switch xu3 to arch timer?
Do you have any info or ideas about enabling MCT G_TCON register to start arch timer
and what's the best way to do it?
Thanks.
Signature · 1 line
Best regards, Klimov Alexey
Hi,
Quoted 32 lines · Alexey Klimov
Hi Doug,
Quoted 20 lines · Doug Anderson
Alexey,
Quoted 13 lines · Alexey Klimov
Hi all,
year(s) ago it was discovered that MCT timer and ARM architectured timer
are the same hardware with different interface. Here [1].I followed mail-list discussions about removing MCT and using arch timer for Exynos5-based SoCs but things aren't moving at least latest upstream kernel on odroid-xu3 will use MCT as default timers. Maybe the reason are some power-management related things that very specific to Samsung. I don't know.
Quoted 3 lines
Idea of this draft patchset comes from Doug patches when he tried to optimize read of 64-bit counter located in mmio. [2]
Why not using cp15 counter instead if possible?Quoted 8 lines
I hate to burst your bubble here, but...
...I think it would be a bad idea to use the cp15 counter on exynos 5422. According to Samsung, there are issues where using cp15 could sometimes return the wrong value, especially if you happen to read it while on an A7 instead of an A15. It will tend to work pretty well, but Samsung claimed that it might not work right on some CPUs or at some temperatures.
Quoted 2 lines
I think this is right time when I can say "If only I had known about it before!".
Yeah, we spent a whole lot of time on this too before giving up after we found out it was not possible to use.
Quoted 2 lines
That's quite interesting especially about temperature issues. Maybe it's worth to insert comments in driver/docs about this.
Maybe not a bad idea. Depends if you think someone else will try to do this... ...and if they do, will they check mailing list traffic.
Quoted 9 lines
Quoted 4 lines
Generally I'd say that if the arch timer works for you: use the arch timer. If the arch timer doesn't work for you, use the MCT.
-Doug
I possess no knowledge if there're any different hw revisions of odroid-xu3 on the field
but looks like arch timer works fine for me on odroid-xu3 Exynos5422 SoC with upstream kernel.
Well, it looked like it worked for us too. In fact, I never saw a failure. All I know is that Samsung said: don't do this, it's not a good idea. The performance gain wasn't enough to go against that... That's when I started trying to optimize the MCT a little bit. ;)
-Doug
Quoted 2 lines · Doug Anderson
Hi,
Hi,
Quoted 40 lines · Alexey Klimov
Quoted 32 lines
Hi Doug,
Quoted 20 lines · Doug Anderson
Alexey,
Quoted 13 lines · Alexey Klimov
Hi all,
year(s) ago it was discovered that MCT timer and ARM architectured timer
are the same hardware with different interface. Here [1].I followed mail-list discussions about removing MCT and using arch timer for Exynos5-based SoCs but things aren't moving at least latest upstream kernel on odroid-xu3 will use MCT as default timers. Maybe the reason are some power-management related things that very specific to Samsung. I don't know.
Quoted 3 lines
Idea of this draft patchset comes from Doug patches when he tried to optimize read of 64-bit counter located in mmio. [2]
Why not using cp15 counter instead if possible?Quoted 8 lines
I hate to burst your bubble here, but...
...I think it would be a bad idea to use the cp15 counter on exynos 5422. According to Samsung, there are issues where using cp15 could sometimes return the wrong value, especially if you happen to read it while on an A7 instead of an A15. It will tend to work pretty well, but Samsung claimed that it might not work right on some CPUs or at some temperatures.
Quoted 2 lines
I think this is right time when I can say "If only I had known about it before!".
Yeah, we spent a whole lot of time on this too before giving up after we found out it was not possible to use.
Quoted 10 lines
Quoted 2 lines
That's quite interesting especially about temperature issues. Maybe it's worth to insert comments in driver/docs about this.
Maybe not a bad idea. Depends if you think someone else will try to do this... ...and if they do, will they check mailing list traffic.
Quoted 3 lines
Quoted 3 lines
Generally I'd say that if the arch timer works for you: use the arch timer. If the arch timer doesn't work for you, use the MCT.
One more thing, if you change it for your own private platform, it's up to you but if not, i.e., for mass product, please don't change it. Because it's not guaranteed even you haven't seen its failure...
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-Doug
I possess no knowledge if there're any different hw revisions of odroid-xu3 on the field
but looks like arch timer works fine for me on odroid-xu3 Exynos5422 SoC with upstream kernel.
Quoted 5 lines
Well, it looked like it worked for us too. In fact, I never saw a failure. All I know is that Samsung said: don't do this, it's not a good idea. The performance gain wasn't enough to go against that... That's when I started trying to optimize the MCT a little bit. ;)
Yeah and it was tough time for me when I discussed about that with hardware designer at that time ... haha...
Thanks, Doug.
- Kukjin