Re: [RFC 0/3] mmc: Add dynamic frequency scaling

[Tomeu Vizoso]

On 12 January 2015 at 10:23, Krzysztof Kozlowski
<k.kozlowski@xxxxxxxxxxx> wrote:
> Hi,
>
>
> I would like to hear some comments about idea of scaling MMC clock
> frequency. The basic idea is to lower the clock when device is
> completely idle or not busy enough.
>
> The patchset adds MMC card as a devfreq device and uses simple_ondemand
> as governor. In idle this gave benefits (less energy consumed during
> idle):
> 1. Trats2 (Exynos4412): 2.6%
> 2. Rinato (Exynos3250): 1%
>
> but (especially on Rinato) it had impact on performance (probably
> because ondemand triggering a little to late). What is interesting
> manually changing the clock (without this patchset) gave slightly
> bigger benefits. Maybe the devfreq introduces noticeable overhead?

Could it be because of the polling interval being too long thus it
being too slow to ramp up?

That's a problem with all polling devfreq drivers, it has been
proposed before using pm_qos to to reduce the polling interval when
some event indicates that the utilization may grow abruptly in the
near future.

I don't think pm_qos is the best mechanism for that, maybe something
new needs to be devised.

Regards,

Tomeu

>
> Comments are welcomed. Maybe on other platforms this has bigger impact?
>
> Best regards,
> Krzysztof
>
>
> Krzysztof Kozlowski (3):
>   mmc: Add dynamic frequency scaling
>   ARM: dts: Specify MSHC realistic clocks and use frequency scaling
>   ARM: dts: Use frequency scaling for MSHC
>
>  Documentation/devicetree/bindings/mmc/mmc.txt |   2 +
>  arch/arm/boot/dts/exynos3250-rinato.dts       |   1 +
>  arch/arm/boot/dts/exynos4412-trats2.dts       |   4 +-
>  drivers/mmc/card/block.c                      | 247 ++++++++++++++++++++++++++
>  drivers/mmc/core/Kconfig                      |  16 ++
>  drivers/mmc/core/core.h                       |   1 -
>  drivers/mmc/core/host.c                       |   2 +
>  include/linux/mmc/card.h                      |   8 +
>  include/linux/mmc/host.h                      |   3 +
>  9 files changed, 282 insertions(+), 2 deletions(-)
>
> --
> 1.9.1
>
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