linux-kernel - Re: [RFC PATCH] checkpatch: check for function calls with struct or union on stack

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Message-Id: <20180726133829.bbe2677217f905c5df11d2d6@linux-foundation.org>
Date:   Thu, 26 Jul 2018 13:38:29 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Andy Whitcroft <apw@...onical.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] checkpatch: check for function calls with struct or
 union on stack

On Thu, 26 Jul 2018 13:05:29 -0700 Joe Perches <joe@...ches.com> wrote:

> On Thu, 2018-07-26 at 12:28 -0700, Andrew Morton wrote:
> > On Thu, 26 Jul 2018 12:25:33 -0700 Andrew Morton <akpm@...ux-foundation.org> wrote:
> > 
> > > I'll give it a spin, see how noisy it is.
> > 
> > Actually, I would prefer if the message, changelog and title
> > used the term "passed by value".  It's a more familiar term
> > and it is possible for a passed-by-value aggregate to in fact 
> > be passed in registers.
> 
> RFC, No worries, I'll change it if it's OK.
> 
> I'm testing it right now against the last 5000 commits
> (which takes awhile here) via
> 
> $ git log --no-merges --format=oneline -5000 | \
>   cut -f1 -d" " | \
>   while read commit ; do \
>     echo $commit; \
>     ./scripts/checkpatch.pl --git $commit --types=aggregate_on_stack --quiet --no-summary ; \
>   done
> 
> It doesn't seem noisy at all, but maybe there are a few
> known structs like "struct timespec64" that could be
> excluded.
> 
> The only real hits so far are:
> 
> commit f2fb56afba11426ee5c9603b28a9827c530909c0
> WARNING: Unusual use of 'struct msm_display_topology' on stack
> #28374: FILE: drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c:149:
> +enum dpu_rm_topology_name
> +dpu_rm_get_topology_name(struct msm_display_topology topology)
> +{

hm.  12 bytes.  I don't know if this would be more efficient than using
const struct msm_display_topology*.

> and
> 
> 33477d84c26bbfa626da2c032e567a90dd70a528
> WARNING: Unusual use of 'struct cppc_perf_fb_ctrs' on stack
> #45: FILE: drivers/cpufreq/cppc_cpufreq.c:307:
> +static int cppc_get_rate_from_fbctrs(struct cppc_cpudata *cpu,
> +				     struct cppc_perf_fb_ctrs fb_ctrs_t0,
> +				     struct cppc_perf_fb_ctrs fb_ctrs_t1)
> +{

Two 32-byte structures?  That seems excessive.

Yes, a warning which sends developers back for a bit more thinnking
sounds useful.