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

[Liming Sun]

> -----Original Message-----
> From: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
> Sent: Friday, August 11, 2023 4:36 AM
> To: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> Cc: Liming Sun <limings@xxxxxxxxxx>; David Thompson
> <davthompson@xxxxxxxxxx>; Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>; linux-
> mmc@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v7] mmc: sdhci-of-dwcmshc: Add runtime PM operations
> 
> On Fri, 11 Aug 2023 at 07:57, Adrian Hunter <adrian.hunter@xxxxxxxxx>
> wrote:
> >
> > On 10/08/23 19:34, Ulf Hansson wrote:
> > > 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.
> >
> > In that case, a matching pm_runtime_put() is needed also at the
> > end of the resume callback.
> 
> Yes, of course. Or depending if we are using the force_suspend|resume
> helper, a pm_runtime_put_noidle is sufficient after
> pm_runtime_force_suspend() has been called.

Thanks Ulf/Adrian! Plan to upload v8 with the following changes:
- Remove pm_runtime_force_suspend/resume() from dwcmshc_suspend()/dwcmshc_resume() (Adrian's comment).
- Add comments for dwcmshc_resume()/dwcmshc_suspend();
(According to Andrian's comment).
- Add pm_runtime_get_sync()/pm_runtime_put() in dwcmshc_suspend(), which is Ulf option-1.  Option-2 seems more efficient, but it involves more changes and I couldn't test the impact on other SoC. Maybe for future enhancement?
> 
> [...]
> 
> Kind regards
> Uffe