linux-kernel - Re: linux-next: build warning after merge of the fbdev tree

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Date:   Tue, 2 Apr 2019 17:16:47 +0200
From:   Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
To:     Tom Li <tomli@...li.me>
Cc:     Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Kees Cook <keescook@...omium.org>, linux-fbdev@...r.kernel.org
Subject: Re: linux-next: build warning after merge of the fbdev tree


On 04/02/2019 02:38 PM, Tom Li wrote:
> On Tue, Apr 02, 2019 at 09:30:07AM +1100, Stephen Rothwell wrote:
>> Hi Bartlomiej,
>>
>> After merging the fbdev tree, today's linux-next build (x86_64
>> allmodconfig) produced this warning:
>>
>> drivers/video/fbdev/sm712fb.c: In function 'smtc_blank':
>> drivers/video/fbdev/sm712fb.c:900:4: warning: this statement may fall through [-Wimplicit-fallthrough=]
>>     smtc_seqw(0x6b, 0x02);
>>     ^~~~~~~~~~~~~~~~~~~~~
>> drivers/video/fbdev/sm712fb.c:901:3: note: here
>>    case 0x720:
>>    ^~~~
>>
>> Introduced by commit
>>
>>   f627caf55b8e ("fbdev: sm712fb: fix crashes and garbled display during DPMS modesetting")
>>
> 
> Nice catch! Thanks!
> 
> This bug was introduced by me while attempting to fix another issue, a result of
> my copy-paste error. Since it only reprograms the clock to a different frequency,
> it's only a benign issue without visible side-effect, so it also evaded Sudip
> Mukherjee's code review and regression tests.
> 
> But what's I'm more concerned here is the failure of scripts/checkpatch.pl. I
> thought ./checkpatch.pl should have caught it, but for some reasons it cannot
> detect this one.
> 
> $ ./scripts/checkpatch.pl 0001-fbdev-sm712fb-fix-crashes-and-garbled-display-during.patch
> total: 0 errors, 0 warnings, 105 lines checked
> 
> So I mistakenly assumed the patch doesn't have a problem... It seems checkpatch.pl
> cannot detect fallthroughs in nested switch/case statements? I'm not sure. Should I
> report it to the maintainers of checkpatch.pl?
> 
> Anyway, please apply the following patch ASAP.
> 
> Thanks,
> Tom Li
> 
>>>From 040fa4e6cc8b338cd845c11fd3efd7394ca55108 Mon Sep 17 00:00:00 2001
> From: Yifeng Li <tomli@...li.me>
> Date: Tue, 2 Apr 2019 20:25:20 +0800
> Subject: [PATCH] fbdev: sm712fb: fix memory frequency by avoiding a
>  switch/case fallthrough.
> 
> A fallthrough in switch/case was introduced in f627caf55b8e ("fbdev:
> sm712fb: fix crashes and garbled display during DPMS modesetting"),
> due to my copy-paste error, which would cause the memory clock frequency
> for SM720 to be programmed to SM712.
> 
> Since it only reprograms the clock to a different frequency, it's only
> a benign issue without visible side-effect, so it also evaded Sudip
> Mukherjee's code review and regression tests. scripts/checkpatch.pl
> also failed to discover the issue, possibly due to nested switch
> statements.
> 
> This issue was found by Stephen Rothwell by building linux-next with
> -Wimplicit-fallthrough.
> 
> Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
> Fixes: f627caf55b8e ("fbdev: sm712fb: fix crashes and garbled display during DPMS modesetting")
> Signed-off-by: Yifeng Li <tomli@...li.me>

Applied to fbdev-for-next, thanks!

> ---
>  drivers/video/fbdev/sm712fb.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/video/fbdev/sm712fb.c b/drivers/video/fbdev/sm712fb.c
> index 1e2503b52c6f..f1dcc6766d1e 100644
> --- a/drivers/video/fbdev/sm712fb.c
> +++ b/drivers/video/fbdev/sm712fb.c
> @@ -898,6 +898,7 @@ static int smtc_blank(int blank_mode, struct fb_info *info)
>  		case 0x712:
>  			smtc_seqw(0x6a, 0x16);
>  			smtc_seqw(0x6b, 0x02);
> +			break;
>  		case 0x720:
>  			smtc_seqw(0x6a, 0x0d);
>  			smtc_seqw(0x6b, 0x02);

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics