Re: [PATCH v3] mmc: sdhci-msm: Warn about overclocking SD/MMC

[Doug Anderson]

Hi,

On Thu, Dec 10, 2020 at 9:56 PM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>
> On 10/12/20 11:27 pm, Douglas Anderson wrote:
> > As talked about in commit 5e4b7e82d497 ("clk: qcom: gcc-sdm845: Use
> > floor ops for sdcc clks"), most clocks handled by the Qualcomm clock
> > drivers are rounded _up_ by default instead of down.  We should make
> > sure SD/MMC clocks are always rounded down in the clock drivers.
> > Let's add a warning in the Qualcomm SDHCI driver to help catch the
> > problem.
> >
> > This would have saved a bunch of time [1].
> >
> > [1] http://lore.kernel.org/r/20201210102234.1.I096779f219625148900fc984dd0084ed1ba87c7f@changeid
> >
> > Suggested-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
> > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > ---
> >
> > Changes in v3:
> > - Proper printf format code.
> >
> > Changes in v2:
> > - Store rate in unsigned long, not unsigned int.
> > - Reuse the clk_get_rate() in the later print.
> >
> >  drivers/mmc/host/sdhci-msm.c | 15 +++++++++++++--
> >  1 file changed, 13 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/mmc/host/sdhci-msm.c b/drivers/mmc/host/sdhci-msm.c
> > index 3451eb325513..50beb407dbe9 100644
> > --- a/drivers/mmc/host/sdhci-msm.c
> > +++ b/drivers/mmc/host/sdhci-msm.c
> > @@ -353,6 +353,7 @@ static void msm_set_clock_rate_for_bus_mode(struct sdhci_host *host,
> >       struct sdhci_msm_host *msm_host = sdhci_pltfm_priv(pltfm_host);
> >       struct mmc_ios curr_ios = host->mmc->ios;
> >       struct clk *core_clk = msm_host->bulk_clks[0].clk;
> > +     unsigned long achieved_rate;
> >       int rc;
> >
> >       clock = msm_get_clock_rate_for_bus_mode(host, clock);
> > @@ -363,10 +364,20 @@ static void msm_set_clock_rate_for_bus_mode(struct sdhci_host *host,
> >                      curr_ios.timing);
> >               return;
> >       }
> > +
> > +     /*
> > +      * Qualcomm clock drivers by default round clock _up_ if they can't
> > +      * make the requested rate.  This is not good for SD.  Yell if we
> > +      * encounter it.
> > +      */
> > +     achieved_rate = clk_get_rate(core_clk);
> > +     if (achieved_rate > clock)
> > +             pr_warn("%s: Card appears overclocked; req %u Hz, actual %lu Hz\n",
> > +                     mmc_hostname(host->mmc), clock, achieved_rate);
>
> How does a warning help?

The actual fixes are made in the clock drivers, but this seems to be a
common problem in Qualcomm clock drivers.  ...so while this doesn't
actually fix the problem, it hopefully calls enough attention that
someone will know to fix it in the clock driver.


> Also, did you consider setting host->mmc->actual_clock

That makes sense, though I think logically it's a separate change.
I'll add it as patch #2 in the series.  It's not as trivial as I would
have thought.  ;-)

-Doug

>
> > +
> >       msm_host->clk_rate = clock;
> >       pr_debug("%s: Setting clock at rate %lu at timing %d\n",
> > -              mmc_hostname(host->mmc), clk_get_rate(core_clk),
> > -              curr_ios.timing);
> > +              mmc_hostname(host->mmc), achieved_rate, curr_ios.timing);
> >  }
> >
> >  /* Platform specific tuning */
> >
>