linux-kernel - Re: [PATCH v5] coresight: etm4x: avoid build failure with unrolled loops

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Date:   Fri, 8 Jul 2022 16:04:10 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Tao Zhang <quic_taozha@...cinc.com>,
        Mike Leach <mike.leach@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>,
        "coresight@...ts.linaro.org" <coresight@...ts.linaro.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: Re: [PATCH v5] coresight: etm4x: avoid build failure with unrolled loops

On Tue, Jun 28, 2022 at 2:40 AM David Laight <David.Laight@...lab.com> wrote:
>
> ...
> > > Regardless of which compiler or compiler options determine whether a
> > > loop can or can't be unrolled, which determines whether
> > > __builtin_constant_p evaluates to true when passed an expression using a
> > > loop induction variable, it is NEVER safe to allow the preprocessor to
> > > construct inline asm like:
> > >    asm volatile (".inst (0x160 + (i * 4))" : "=r"(__val));
> > >                                   ^ expected constant expression
>
> Can't you use (IIRC) an "=i" constraint with the C expression
> so that the compiler evaluates the expression and passes the
> correct constant value to the assembler?

Yes, though I think it may be even simpler for me to just use
__is_constexpr from include/linux/const.h here than try to rewrite the
existing macro soup to avoid calls to read_sysreg_s. Will send a
follow up.

diff --git a/drivers/hwtracing/coresight/coresight-etm4x.h
b/drivers/hwtracing/coresight/coresight-etm4x.h
index 33869c1d20c3..a7bfea31f7d8 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x.h
+++ b/drivers/hwtracing/coresight/coresight-etm4x.h
@@ -7,6 +7,7 @@
 #define _CORESIGHT_CORESIGHT_ETM_H

 #include <asm/local.h>
+#include <linux/const.h>
 #include <linux/spinlock.h>
 #include <linux/types.h>
 #include "coresight-priv.h"
@@ -515,7 +516,7 @@
        ({
         \
                u64 __val;
         \

         \
-               if (__builtin_constant_p((offset)))
         \
+               if (__is_constexpr((offset)))
         \
                        __val =
read_etm4x_sysreg_const_offset((offset));       \
                else
         \
                        __val = etm4x_sysreg_read((offset), true,
(_64bit));    \

--
Thanks,
~Nick Desaulniers