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

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Date:   Thu, 25 Feb 2021 21:08:58 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Mike Leach <mike.leach@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        coresight@...ts.linaro.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] coresight: etm4x: work around clang-12+ build failure

On Thu, Feb 25, 2021 at 5:45 PM Mathieu Poirier
<mathieu.poirier@...aro.org> 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