linux-kernel - Re: [PATCH V1 2/7] mmc: core: devfreq: Add devfreq based clock scaling support

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Date:   Wed, 17 Oct 2018 09:04:52 -0500
From:   Rob Herring <robh@...nel.org>
To:     Sayali Lokhande <sayalil@...eaurora.org>
Cc:     adrian.hunter@...el.com, ulf.hansson@...aro.org,
        mark.rutland@....com, linux-mmc@...r.kernel.org,
        linux-kernel@...r.kernel.org, shawn.lin@...k-chips.com,
        linux-arm-msm@...r.kernel.org, georgi.djakov@...aro.org,
        devicetree@...r.kernel.org, asutoshd@...eaurora.org,
        stummala@...eaurora.org, venkatg@...eaurora.org,
        vviswana@...eaurora.org, bjorn.andersson@...aro.org,
        riteshh@...eaurora.org, vbadigan@...eaurora.org,
        dianders@...gle.com, Talel Shenhar <tatias@...eaurora.org>
Subject: Re: [PATCH V1 2/7] mmc: core: devfreq: Add devfreq based clock
 scaling support

On Mon, Oct 08, 2018 at 06:28:01PM +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.

Is there really power savings if there's no voltage control?

> 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.

Why the short lines?

> 
> Signed-off-by: Talel Shenhar <tatias@...eaurora.org>
> Signed-off-by: Sayali Lokhande <sayalil@...eaurora.org>
> ---
>  .../devicetree/bindings/mmc/sdhci-msm.txt          |  10 +

Bindings should be separate patches.

>  drivers/mmc/core/core.c                            | 556 +++++++++++++++++++++
>  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, 1055 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.

Why is this qcom specific?

I believe I also saw interconnect binding for SD/MMC. How does that 
relate to this? There should be some coordination of this work.

>  Example:
>  
>  	sdhc_1: sdhci@...24900 {
> @@ -43,6 +52,7 @@ Example:
>  
>  		clocks = <&gcc GCC_SDCC1_APPS_CLK>, <&gcc GCC_SDCC1_AHB_CLK>;
>  		clock-names = "core", "iface";
> +		qcom,devfreq,freq-table = <52000000 200000000>;
>  	};
>  
>  	sdhc_2: sdhci@...a4900 {