Re: [PATCH] coresight: etm4x: work around clang-12+ build failure

[Arnd Bergmann]

On Thu, Feb 25, 2021 at 5:45 PM Mathieu Poirier
<mathieu.poirier@xxxxxxxxxx> wrote:
> On Thu, Feb 25, 2021 at 10:42:58AM +0100, Arnd Bergmann wrote:
> > index 15016f757828..4cccf874a602 100644
> > --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
> > +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> > @@ -691,13 +691,13 @@ static void etm4_disable_hw(void *info)
> >                       "timeout while waiting for PM stable Trace Status\n");
> >
> >       /* read the status of the single shot comparators */
> > -     for (i = 0; i < drvdata->nr_ss_cmp; i++) {
> > +     for (i = 0; i < min_t(u32, drvdata->nr_ss_cmp, ETM_MAX_SS_CMP); i++) {
> >               config->ss_status[i] =
> >                       etm4x_relaxed_read32(csa, TRCSSCSRn(i));
> >       }
> >
> >       /* read back the current counter values */
> > -     for (i = 0; i < drvdata->nr_cntr; i++) {
> > +     for (i = 0; i < min_t(u32, drvdata->nr_cntr, ETMv4_MAX_CNTR); i++) {
>
> This patch will work and I'd be happy to apply it if this was the only instance,
> but there are dozens of places in the coresight drivers where such patterns are
> used.  Why are those not flagged as well?  And shouldn't the real fix be with
> clang?

This is the only one I see in randconfig build tests. In fact I only actually
see the second one of the two causing problems, but it seemed silly
to change only one of them. I had not noticed the other similar
loops that use etm4x_read32() instead of etm4x_relaxed_read32().

I first suspected that the memory clobber in  __iormb() prevents the
compiler from running into the same problem there, but changing all
etm4x_read32() to etm4x_relaxed_read32() did not make it show up
in other places either.

While I expect this will be fixed in mainline clang, the workaround should
be harmless and let us use the existing releases as well.

      Arnd