Re: [PATCH v7] mmc: sdhci-of-dwcmshc: Add runtime PM operations

[Ulf Hansson]

On Thu, 10 Aug 2023 at 14:44, Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>
> On 10/08/23 13:21, Ulf Hansson wrote:
> > On Thu, 10 Aug 2023 at 10:13, Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
> >>
> >> On 8/08/23 23:23, Liming Sun wrote:
> >>> This commit implements the runtime PM operations to disable eMMC
> >>> card clock when idle.
> >>>
> >>> Reviewed-by: David Thompson <davthompson@xxxxxxxxxx>
> >>> Signed-off-by: Liming Sun <limings@xxxxxxxxxx>
> >>> ---
> >>> v6->v7:
> >>>     - Address Ulf's comment;
> >>> v5->v6:
> >>>     - Address Adrian's more comments and add coordination between
> >>>       runtime PM and system PM;
> >>> v4->v5:
> >>>     - Address Adrian's comment to move the pm_enable to the end to
> >>>       avoid race;
> >>> v3->v4:
> >>>     - Fix compiling reported by 'kernel test robot';
> >>> v2->v3:
> >>>     - Revise the commit message;
> >>> v1->v2:
> >>>     Updates for comments from Ulf:
> >>>     - Make the runtime PM logic generic for sdhci-of-dwcmshc;
> >>> v1: Initial version.
> >>> ---
> >>>  drivers/mmc/host/sdhci-of-dwcmshc.c | 72 ++++++++++++++++++++++++++++-
> >>>  1 file changed, 70 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/drivers/mmc/host/sdhci-of-dwcmshc.c b/drivers/mmc/host/sdhci-of-dwcmshc.c
> >>> index e68cd87998c8..c8e145031429 100644
> >>> --- a/drivers/mmc/host/sdhci-of-dwcmshc.c
> >>> +++ b/drivers/mmc/host/sdhci-of-dwcmshc.c
> >>> @@ -15,6 +15,7 @@
> >>>  #include <linux/module.h>
> >>>  #include <linux/of.h>
> >>>  #include <linux/of_device.h>
> >>> +#include <linux/pm_runtime.h>
> >>>  #include <linux/reset.h>
> >>>  #include <linux/sizes.h>
> >>>
> >>> @@ -548,9 +549,13 @@ static int dwcmshc_probe(struct platform_device *pdev)
> >>>
> >>>       host->mmc->caps |= MMC_CAP_WAIT_WHILE_BUSY;
> >>>
> >>> +     pm_runtime_get_noresume(dev);
> >>> +     pm_runtime_set_active(dev);
> >>> +     pm_runtime_enable(dev);
> >>> +
> >>>       err = sdhci_setup_host(host);
> >>>       if (err)
> >>> -             goto err_clk;
> >>> +             goto err_rpm;
> >>>
> >>>       if (rk_priv)
> >>>               dwcmshc_rk35xx_postinit(host, priv);
> >>> @@ -559,10 +564,15 @@ static int dwcmshc_probe(struct platform_device *pdev)
> >>>       if (err)
> >>>               goto err_setup_host;
> >>>
> >>> +     pm_runtime_put(dev);
> >>> +
> >>>       return 0;
> >>>
> >>>  err_setup_host:
> >>>       sdhci_cleanup_host(host);
> >>> +err_rpm:
> >>> +     pm_runtime_disable(dev);
> >>> +     pm_runtime_put_noidle(dev);
> >>>  err_clk:
> >>>       clk_disable_unprepare(pltfm_host->clk);
> >>>       clk_disable_unprepare(priv->bus_clk);
> >>> @@ -606,6 +616,12 @@ static int dwcmshc_suspend(struct device *dev)
> >>>       if (ret)
> >>>               return ret;
> >>>
> >>> +     ret = pm_runtime_force_suspend(dev);
> >>> +     if (ret) {
> >>> +             sdhci_resume_host(host);
> >>> +             return ret;
> >>> +     }
> >>
> >> Since you are only using the runtime PM callbacks to turn off the card
> >> clock via SDHCI_CLOCK_CONTROL, pm_runtime_force_suspend() and
> >> pm_runtime_force_resume() are not needed at all.
> >
> > Right, it can be done without these too.
> >
> >>
> >> sdhci_suspend_host() does not care if SDHCI_CLOCK_CARD_EN is on or off.
> >> (And you are disabling pltfm_host->clk and priv->bus_clk, so presumably
> >> the result is no clock either way)
> >>
> >> sdhci_resume_host() does not restore state unless
> >> SDHCI_QUIRK2_HOST_OFF_CARD_ON is used, it just resets, so the internal clock
> >> SDHCI_CLOCK_INT_EN is off which is consistent with either runtime suspended
> >> or runtime resumed.
> >
> > Even if this may work, to me, it doesn't look like good practice for
> > how to use runtime PM in combination with system wide suspend/resume.
> >
> > The point is, sdhci_suspend|resume_host() may end up reading/writing
> > to sdhci registers - and we should *not* allow that (because it may
> > not always work), unless the sdhci controller has been runtime resumed
> > first, right?
>
> I am OK with drivers that just want to use runtime PM to turn off a
> functional clock.  sdhci-tegra.c is also doing that although using the
> clock framework.

Yes, I agree. At least this works for SoC specific drivers.

>
> Certainly that approach assumes that the host controller's power state
> is not changed due to runtime PM.
>
> To ensure that the host controller is runtime resumed before calling
> sdhci_suspend_host(), we can just call pm_runtime_resume() I think.

Yes, that was kind of what I proposed in the other thread as option 1)
(except for the replacement of pm_runtime_force_suspend|resume).

Although, to be clear I would probably use pm_runtime_get_sync()
instead, to make sure the usage count is incremented too.

I don't have a strong opinion here, but from an optimization point of
view I would at least consider what I proposed in option 2) (in the
other thread). The benefit is that it can allow us to potentially
avoid runtime resuming the device, during system suspend.

Kind regards
Uffe