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

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Date:   Tue, 15 Jun 2021 17:30:39 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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

[...]

> >
> >>
> >> In all honesty, I am a bit surprised that the Linux device driver model
> >> does not try to default the absence of a ->shutdown() to a ->suspend()
> >> call since in most cases they are functionally equivalent, or should be,
> >> in that they need to save power and quiesce the hardware, or leave
> >> enough running to support a wake-up event.
> >
> > Well, the generall assumption is that the platform is going to be
> > entirely powered off, thus moving things into a low power state would
> > just be a waste of execution cycles. Of course, that's not the case
> > for your platform.
>
> That assumption may hold true for ACPI-enabled machines but power off is
> offered as a general function towards other more flexible and snowflaky
> systems (read embedded) as well.
>
> >
> > As I have stated earlier, to me it looks a bit questionable to use the
> > kernel_power_off() path to support the use case you describe. On the
> > other hand, we may not have a better option at this point.
>
> Correct, there is not really anything better and I am not sure what the
> semantics of something better could be anyway.
>
> >
> > Just a few things, from the top of my head, that we certainly are
> > missing to support your use case through kernel_power_off() path
> > (there are certainly more):
> > 1. In general, subsystems/drivers don't care about moving things into
> > lower power modes from their ->shutdown() callbacks.
> > 2. System wakeups and devices being affected in the wakeup path, needs
> > to be respected properly. Additionally, userspace should be able to
> > decide if system wakeups should be enabled or not.
> > 3. PM domains don't have ->shutdown() callbacks, thus it's likely that
> > they remain powered on.
> > 4. Etc...
>
> For the particular eMMC driver being discussed here this is a no-brainer
 > because  it is not a wake-up source, therefore there is no reason not to
> power if off if we can. It also seems proper to have it done by the
> kernel as opposed to firmware.

Okay, I have applied the $subject patch onto my next branch, along
with patch 1/2 (the DT doc change).

However, I still think we should look for a proper long term solution,
because the kernel_power_off() path does not currently support your
use case, with system wakeups etc.

I guess it could be a topic that is easier to bring up at the Linux
Plumbers Conf, for example.

Kind regards
Uffe