Re: [PATCH V4] mmc: sdhci-msm: Update system suspend/resume callbacks of sdhci-msm platform driver

[Ulf Hansson <ulf.hansson@linaro.org>]

On Thu, 19 Mar 2020 at 18:42, Stephen Boyd <swboyd@chromium.org> wrote:
>
> Quoting Ulf Hansson (2020-03-06 02:07:41)
> > On Wed, 4 Mar 2020 at 17:46, Stephen Boyd <swboyd@chromium.org> wrote:
> > >
> > > Quoting Ulf Hansson (2020-03-04 07:34:29)
> > > > On Thu, 20 Feb 2020 at 07:45, Shaik Sajida Bhanu <sbhanu@codeaurora.org> wrote:
> > > > >
> > > > > The existing suspend/resume callbacks of sdhci-msm driver are just
> > > > > gating/un-gating the clocks. During suspend cycle more can be done
> > > > > like disabling controller, disabling card detection, enabling wake-up events.
> > > > >
> > > > > So updating the system pm callbacks for performing these extra
> > > > > actions besides controlling the clocks.
> > > > >
> > > > > Signed-off-by: Shaik Sajida Bhanu <sbhanu@codeaurora.org>
> > > > > Reviewed-by: Stephen Boyd <swboyd@chromium.org>
> > > > > ---
> [...]
> >
> > >
> > > >
> > > > > +
> > > > > +       ret = pm_runtime_force_suspend(dev);
> > > >
> > > > It looks to me that perhaps you could make use of solely
> > > > pm_runtime_force_suspend(), then just skip calling
> > > > sdhci_suspend|resume_host() altogether. Do you think that could work?
> > >
> > > Does that do all the things the commit text mentions is desired for
> > > system suspend?
> >
> > No. :-)
> >
> > But why is system wakeup needed for an eMMC card?
> >
>
> I don't know if system wakeup is needed for an eMMC card. Probably only
> if you plug in a card and some daemon wants to wake up and probe the
> card for auto-play or something like that? Seems possible so might as
> well expose the CD gpio as a wakeup in that case and let userspace
> decide if it wants to do that.

Right, card detect IRQs could be useful for system wakeups.

I assume you are using a GPIO IRQ for that, which is easily managed,
as the runtime PM status of the mmc controller is irrelevant when
configuring the GPIO IRQ as wakeup.

We even have a helper for doing this, mmc_gpio_set_cd_wake().

>
> Is runtime suspended state the same as system suspended state here
> though? The commit text seems to imply that only clks are disabled when
> it's desirable to disable the entire controller. I'm still fuzzy on how
> runtime PM and system PM interact because it seems to have changed since
> I looked last a few years ago. If the driver can stay in a runtime
> suspended state across system suspend then I'm all for it. That would
> save time for system PM transitions.

In most cases this should be possible. And so far, for this case, I
haven't found a good reason to why it shouldn't work.

Although, perhaps we need to improve some of the sdhci's library
functions for PM, to better support this.

Kind regards
Uffe