linux-kernel - Re: drivers/gpu/drm/panel/panel-samsung-ld9040.c:240:12: warning: stack frame size of 8312 bytes in function 'ld9040

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Date:   Sat, 27 Jun 2020 22:43:01 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     kernel test robot <lkp@...el.com>,
        Thierry Reding <thierry.reding@...il.com>, sam@...nborg.org,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        kbuild-all@...ts.01.org,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>
Subject: Re: drivers/gpu/drm/panel/panel-samsung-ld9040.c:240:12: warning:
 stack frame size of 8312 bytes in function 'ld9040_prepare'

Hi Nick,

On Mon, 22 Jun 2020 at 19:50, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>

> > I really don't get what's the problem here. The listing of
> > ld9040_prepare at the given commit and with the given .config is:
>
> I wrote a tool to help debug these.
> https://github.com/ClangBuiltLinux/frame-larger-than
> If you fetch the randconfig and rebuild with debug info, that tool
> will help show you which variables are used in which stack frames and
> what their sizes are.  Also note that strange things get dug up from
> randconfigs.
>
>
> --
> Thanks,
> ~Nick Desaulniers

I ran your tool and it basically told me that all 11 calls to
ld9040_dcs_write from within ld9040_init are inlined by the compiler.
Each of these ld9040_dcs_write functions calls ld9040_spi_write_word
twice, so 22 inline calls to that. Now, sizeof(struct
spi_transfer)=136 and sizeof(struct spi_message)=104, so, no wonder we
run out of stack pretty quickly.

But my question is: what's wrong with the code, if anything at all?
Why does the compiler try to inline it, and then complain that it's
using too much stack when basically nobody appears to have asked it to
inline it?

Thanks,
-Vladimir