linux-kernel - Re: [PATCH] mmc: core: limit probe clock frequency to configured f

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Date:   Fri, 17 Jan 2020 16:26:30 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mmc: core: limit probe clock frequency to configured f_max

On Fri, 17 Jan 2020 at 15:05, Michał Mirosław <mirq-linux@...e.qmqm.pl> wrote:
>
> On Thu, Jan 16, 2020 at 03:07:22PM +0100, Ulf Hansson wrote:
> > On Thu, 2 Jan 2020 at 11:54, Michał Mirosław <mirq-linux@...e.qmqm.pl> wrote:
> > >
> > > Currently MMC core disregards host->f_max during card initialization
> > > phase. Obey upper boundary for the clock frequency and skip faster
> > > speeds when they are above the limit.
> >
> > Is this a hypothetical problem or a real problem?
>
> This is a problem on noisy or broken boards or cards - so needed for
> debugging such a combination. I wouldn't expect this is required for
> normal devices.

Alright.

>
> > > Signed-off-by: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> > > ---
> > >  drivers/mmc/core/core.c | 10 ++++++++--
> > >  1 file changed, 8 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
> > > index abf8f5eb0a1c..aa54d359dab7 100644
> > > --- a/drivers/mmc/core/core.c
> > > +++ b/drivers/mmc/core/core.c
> > > @@ -2330,7 +2330,13 @@ void mmc_rescan(struct work_struct *work)
> > >         }
> > >
> > >         for (i = 0; i < ARRAY_SIZE(freqs); i++) {
> > > -               if (!mmc_rescan_try_freq(host, max(freqs[i], host->f_min)))
> > > +               unsigned int freq = freqs[i];
> > > +               if (freq > host->f_max) {
> > > +                       if (i + 1 < ARRAY_SIZE(freqs))
> > > +                               continue;
> > > +                       freq = host->f_max;
> >
> > This looks wrong to me. For example, what if f_max = 250KHz and f_min = 50 KHz.
> >
> > Then we should try with 250KHz, then 200KHz and then 100KHz. This
> > isn't what the above code does, I think.
> >
> > Instead it will try with 200KHz and then 100KHz, thus skip 250KHz.
> >
> > Maybe we should figure out what index of freqs[] to start the loop for
> > (before actually starting the loop), depending on the value of f_max -
> > rather than always start at 0.
>
> Yes, it will skip higher frequencies. I didn't view it a problem,
> because the new code guarantees at least one frequency will be tried.
> The eMMC standard specifies only max init frequency (400kHz), so all we
> should try is something less whatever works.
>
> SD spec specifies minimal frequency (100kHz), but I wouldn't expect
> this to be enforced nor required to be anywhere.

Well, my point isn't so much about the specs, rather about providing a
consistent behaviour.

We deal with f_min constraints like I described above, then I think we
should make f_max behave the similar way.

Kind regards
Uffe