Re: [PATCH] mmc: core: limit probe clock frequency to configured f_max

[Michał Mirosław]

On Fri, Jan 17, 2020 at 04:26:30PM +0100, Ulf Hansson wrote:
> On Fri, 17 Jan 2020 at 15:05, Michał Mirosław <mirq-linux@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > > > ---
> > > >  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.

Okay, this would be a second fix as trying the same freq multiple times
is not what this code is supposed to do.

Best Regards,
Michał Mirosław