linux-kernel - Re: [PATCH 2/2] mmc: sdhci-iproc: Add support for the legacy sdhci controller on the BCM7211

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Date:   Wed, 9 Jun 2021 16:59:00 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Ulf Hansson <ulf.hansson@...aro.org>,
        Florian Fainelli <f.fainelli@...il.com>
Cc:     Al Cooper <alcooperx@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>,
        DTML <devicetree@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        Ray Jui <rjui@...adcom.com>, Rob Herring <robh+dt@...nel.org>,
        Scott Branden <sbranden@...adcom.com>
Subject: Re: [PATCH 2/2] mmc: sdhci-iproc: Add support for the legacy sdhci
 controller on the BCM7211



On 6/9/2021 2:22 AM, Ulf Hansson wrote:
> On Wed, 9 Jun 2021 at 05:07, Florian Fainelli <f.fainelli@...il.com> wrote:
>>
>>
>>
>> On 6/8/2021 5:40 AM, Ulf Hansson wrote:
>>> On Wed, 2 Jun 2021 at 21:28, Al Cooper <alcooperx@...il.com> wrote:
>>>>
>>>> Add support for the legacy Arasan sdhci controller on the BCM7211 and
>>>> related SoC's. This includes adding a .shutdown callback to increase
>>>> the power savings during S5.
>>>
>>> Please split this into two separate changes.
>>>
>>> May I also ask about the ->shutdown() callback and in relation to S5.
>>> What makes the ->shutdown callback only being invoked for S5?
>>
>> It is not only called for S5 (entered via poweroff on a prompt) but also
>> during kexec or reboot. The poweroff path is via:
>>
>> kernel_power_off() -> kernel_shutdown_prepare() -> device_shutdown() ->
>> .shutdown()
>>
>> For kexec or reboot we do not really care about power savings since we
>> are about to load a new image anyway, however for S5/poweroff we do care
>> about quiescing the eMMC controller in a way that its clocks and the
>> eMMC device can be put into low power mode since we will stay in that
>> mode for seconds/hours/days until someone presses a button on their
>> remote (or other wake-up sources).
> 
> Hmm, I am not sure I understand correctly. At shutdown we don't care
> about wake-up sources from the kernel point of view, instead we treat
> everything as if it will be powered off.

The same .shutdown() path is used whether you kexec, reboot or poweroff,
but for poweroff we do care about allowing specific wake-up sources
configured as such to wake-up the system at a later time, like GPIOs,
RTC, etc.

> 
> We put devices into low power state at system suspend and potentially
> also during some of the hibernation phases.
> 
> Graceful shutdown of the eMMC is also managed by the mmc core.

AFAICT that calls mmc_blk_shutdown() but that is pretty much it, the
SDHCI platform_driver still needs to do something in order to conserve
power including disabling host->clk, otherwise we would not have done
that for sdhci-brcmstb.c.
-- 
Florian