linux-kernel - Re: [PATCH] drm/i915/tv: avoid possible division by zero

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Date:   Tue, 18 Jul 2023 09:13:11 +0800
From:   Su Hui <suhui@...china.com>
To:     Andrzej Hajda <andrzej.hajda@...el.com>,
        jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
        rodrigo.vivi@...el.com, tvrtko.ursulin@...ux.intel.com,
        airlied@...il.com, daniel@...ll.ch, nathan@...nel.org,
        ndesaulniers@...gle.com, trix@...hat.com
Cc:     ville.syrjala@...ux.intel.com, mripard@...nel.org,
        ankit.k.nautiyal@...el.com, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] drm/i915/tv: avoid possible division by zero

On 2023/7/17 22:52, Andrzej Hajda wrote:
>
>
> On 17.07.2023 08:22, Su Hui wrote:
>> Clang warning: drivers/gpu/drm/i915/display/intel_tv.c:
>> line 991, column 22 Division by zero.
>> Assuming tv_mode->oversample=1 and (!tv_mode->progressive)=1,
>> then division by zero will happen.
>>
>> Fixes: 1bba5543e4fe ("drm/i915: Fix TV encoder clock computation")
>> Signed-off-by: Su Hui <suhui@...china.com>
>> ---
>>   drivers/gpu/drm/i915/display/intel_tv.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/i915/display/intel_tv.c 
>> b/drivers/gpu/drm/i915/display/intel_tv.c
>> index 36b479b46b60..82b54af51f23 100644
>> --- a/drivers/gpu/drm/i915/display/intel_tv.c
>> +++ b/drivers/gpu/drm/i915/display/intel_tv.c
>> @@ -988,7 +988,8 @@ intel_tv_mode_to_mode(struct drm_display_mode *mode,
>>                 const struct tv_mode *tv_mode,
>>                 int clock)
>>   {
>> -    mode->clock = clock / (tv_mode->oversample >> 
>> !tv_mode->progressive);
>> +    mode->clock = clock / (tv_mode->oversample != 1 ?
>> +            tv_mode->oversample >> !tv_mode->progressive : 1);
>
> Seems too smart to me, why not just:
> mode->clock = clock / tv_mode->oversample;
> if (!tv_mode->progressive)
>     mode->clock <<= 1;
> Or trying being smart:
> mode->clock = clock / tv_mode->oversample << !tv_mode->progressive;

Hi,

Yes, this is more readable and clear.
Thanks four your advice. I will send v2 soon.

Su Hui

>
> Btw in both cases there is assumption tv_mode->oversample != 0, I 
> guess it is true.
>
> Regards
> Andrzej
>
>>         /*
>>        * tv_mode horizontal timings:
>