Hello Bartlomiej and Lorenzo,

Thanks a lot for your explanations.

On 08/27/2015 06:58 PM, Bartlomiej Zolnierkiewicz wrote:

Hi,

On Tuesday, August 25, 2015 05:09:32 PM Lorenzo Pieralisi wrote:

quoted

On Tue, Aug 25, 2015 at 03:35:29PM +0100, Bartlomiej Zolnierkiewicz wrote:

quoted

[ added Lorenzo and linux-pm to Cc: ]

Hi,

On Tuesday, August 25, 2015 11:43:38 AM Javier Martinez Canillas wrote:

quoted

[adding Kevin Hilman as cc who was also interested in CPUidle for Exynos]

Hello Krzysztof,

On 08/23/2015 03:26 AM, Krzysztof Kozlowski wrote:

[snip]

quoted

2015-08-21 16:21 GMT+09:00 Javier Martinez Canillas [off-list ref]:

The big.LITTLE cpuidle driver is not a typical Exynos cpuidle driver.
It only executes CPU suspend on a cluster which essentially is a power
down operation.

You are correct, looking at the the big.LITTLE CPUidle driver I see that
it only has two C-states: C0 (normal WFI) and C1 (single CPU power-down)
which as you said, places the CPU into power-down mode by using the MCPM
infrastructure so it's basically a CPU suspend AFAIU.

So what you are saying is that there are deeper C-states supported by the
Exynos 542x SoC family but these are not handled by the b.L CPUidle driver.
 

quoted

When we talk about cpuidle on Exynos, we have in mind one of sleep
modes: AFTR or LPA (sometimes instead of LPA there is LPD or W-AFTR).
Actually this is more like a system idle mode, not CPU idle. The power
savings are much bigger than disabling only one cluster.

Interesting, I was not aware of AFTR and LPA but I looked in the manual now.
Thanks a lot for the information.

I see that the Exynos CPUidle driver (drivers/cpuidle/cpuidle-exynos.c) also
has only two C-states (WFI and C1) but C1 makes the system to enter in AFTR
(system-level power gating).

This is similar to what the downstream ChromiumOS 3.8 kernel CPUidle driver
does IIUC [0].

Yes but upstream does it in a clean way, has support for platforms
requiring secure firmware operations and also implements coupled
AFTR mode on a few platforms.

quoted

quoted

So the question is still valid - whether someone wants or plans to
implement cpuidle for Exynos 542x family. Odroid XU3 is not a priority
here because energy consumption is not an issue there. This is not a
mobile device.

That's true but it will be interesting for the 5420 and 5800 based
Chromebooks since optimizing power consumption would be useful there.

I would be happy to help with reviewing patches etc. but personally
I don't have any plans for doing this work.  I may look into adding
support for newer ARM64 SoCs (Exynos5433) if I find some extra time
(quite unlikely currently).

quoted

I thought that big.LITTLE platforms were encouraged to use the generic b.L
CPUidle driver just like DT platforms should use the generic CPUFreq DT
driver but I guess I misunderstood.

So the b.L CPUidle driver is only to have minimum CPUidle support but a SoC
specific driver is needed to fine tune and get most out of the platform?

Or should the b.L CPUidle driver be extended to add per platform C-states?

This is a good question. Daniel/Lorenzo?

To move the b.L driver to multiple C-states we should first convert it to
the generic CPUidle driver (by defining an MCPM enable-method):

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/cpuidle/cpuidle-arm.c?id=refs/tags/v4.2-rc8

Then we have to figure out how to determine how many CPUidle drivers we
have to create (since idle states are different on different CPUs), since
using the MIDR does not really scale.

For certain I won't support coupled C-states in the DT idle states code,
and every platform requiring them is considered buggy and not worth
merging in the mainline kernel from now onwards, HW should be fixed,
eventually, I am not willing to see code like
drivers/cpuidle/cpuidle-exynos.c in the mainline kernel anymore,
I am sorry.

For Exynos chipsets coupled C-states are not strictly required for
having cpuidle support but make it more useful.  On Exynos chipsets
secondary CPUs must be disabled to allow system to go into deeper
idle modes and this is assured by using coupled C-states.  Original
Android drivers don't use coupled C-states and depend on the rest
of the system to offline secondary CPUs when not needed.

I would of course prefer not to have handle this in software and
be automatically handled by hardware/firmware but that doesn't mean
that we should be declining mainline support for "ugly" hardware
(this has never been "the Linux way" of doing things).

Coming back to the platforms in question (ARM32 big.LITTLE based
Odroid XU3 boards etc.) they have been shipped long time ago and
the best we can do nowadays is to support them as well as possible.

I agree.
 

If somebody wants to implement a separate Exynos542x/Exynos5800
big.LITTLE cpuidle driver for them I see no problem with it and I'm
willing to help in maintaining it.

Ok, I'll see if I can take a look what is needed to implement a Exynos542x CPUidle
driver. I'm quite busy with other stuff right now but I should be less busy in a
couple of weeks.

Maybe is a little bit out of topic but since Anand also asked about CPUFreq support,
are you planning on re-posting your "cpufreq: add generic cpufreq driver support
for Exynos5250/5800 platforms" [0] series?
 

quoted

I think it is time to draw a line with CPUidle on ARM, take stock, clean
it up and find a way forward, current situation is a potpourri of solutions
that all differ in a slightly incompatible way.

Support on ARM64 SoC must be PSCI based and for that there is already
support in the mainline kernel (through the PSCI back-end and the DT
idle states bindings).

I hope that on ARM64 PSCI will be used.  However I have no access to
chipset designers / original firmware authors so I can't tell for
sure.

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics

[0]: https://lkml.org/lkml/2015/4/21/520

Best regards,
-- 
Javier Martinez Canillas
Open Source Group
Samsung Research America