Re: [PATCH RFC 2/7] mmc: core: devfreq: Add devfreq based clock scaling support

[Sayali Lokhande <sayalil@codeaurora.org>]

Hi Rob,

On 7/20/2018 8:54 PM, Rob Herring wrote:
> On Fri, Jul 13, 2018 at 03:22:58PM +0530, Sayali Lokhande wrote:
>> This change adds the use of devfreq to MMC.
>> Both eMMC and SD card will use it.
>> For some workloads, such as video playback, it isn't
>> necessary for these cards to run at high speed.
>> Running at lower frequency, for example 52MHz, in such
>> cases can still meet the deadlines for data transfers.
>> Scaling down the clock frequency dynamically has power
>> savings not only because the bus is running at lower frequency
>> but also has an advantage of scaling down the system core
>> voltage, if supported.
>> Provide an ondemand clock scaling support similar to the
>> cpufreq ondemand governor having two thresholds,
>> up_threshold and down_threshold to decide whether to
>> increase the frequency or scale it down respectively.
>> The sampling interval is in the order of milliseconds.
>> If sampling interval is too low, frequent switching of
>> frequencies can lead to high power consumption and if
>> sampling interval is too high, the clock scaling logic
>> would take long time to realize that the underlying
>> hardware (controller and card) is busy and scale up
>> the clocks.
>>
>> Signed-off-by: Talel Shenhar <tatias@codeaurora.org>
>> Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>
>> ---
>>   .../devicetree/bindings/mmc/sdhci-msm.txt          |  10 +
>>   drivers/mmc/core/core.c                            | 560 +++++++++++++++++++++
>>   drivers/mmc/core/core.h                            |   7 +
>>   drivers/mmc/core/debugfs.c                         |  46 ++
>>   drivers/mmc/core/host.c                            |   8 +
>>   drivers/mmc/core/mmc.c                             | 200 +++++++-
>>   drivers/mmc/core/sd.c                              |  72 ++-
>>   drivers/mmc/host/sdhci-msm.c                       |  37 ++
>>   drivers/mmc/host/sdhci-pltfm.c                     |  11 +
>>   drivers/mmc/host/sdhci.c                           |  27 +
>>   drivers/mmc/host/sdhci.h                           |   8 +
>>   include/linux/mmc/card.h                           |   5 +
>>   include/linux/mmc/host.h                           |  70 +++
>>   13 files changed, 1059 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/devicetree/bindings/mmc/sdhci-msm.txt b/Documentation/devicetree/bindings/mmc/sdhci-msm.txt
>> index 502b3b8..bd8470a 100644
>> --- a/Documentation/devicetree/bindings/mmc/sdhci-msm.txt
>> +++ b/Documentation/devicetree/bindings/mmc/sdhci-msm.txt
>> @@ -26,6 +26,15 @@ Required properties:
>>   	"cal"	- reference clock for RCLK delay calibration (optional)
>>   	"sleep"	- sleep clock for RCLK delay calibration (optional)
>>   
>> +Optional Properties:
>> +- qcom,devfreq,freq-table - specifies supported frequencies for clock scaling.
>> +				    Clock scaling logic shall toggle between these frequencies based
>> +				    on card load. In case the defined frequencies are over or below
>> +				    the supported card frequencies, they will be overridden
>> +				    during card init. In case this entry is not supplied,
>> +				    the driver will construct one based on the card
>> +				    supported max and min frequencies.
>> +				    The frequencies must be ordered from lowest to highest.
> This should be a common binding in which case will just a list of
> frequencies be sufficient.
[Sayali] : Can you please elaborate more on what do you mean by common 
binding here ? Like if you are referring to any already available 
property (by devfreq ?) to pass min-max frequencies ?
Any Documentation file to refer ?
>
> Rob