linux-kernel - Re: [Intel-gfx] [PATCH v2] drm/i915/tv: avoid possible division by zero

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Message-ID: <352f7c10-3592-4a18-abdc-ab554a992d10@nfschina.com>
Date:   Tue, 25 Jul 2023 12:06:50 +0800
From:   Su Hui <suhui@...china.com>
To:     Andi Shyti <andi.shyti@...ux.intel.com>
Cc:     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, andrzej.hajda@...el.com,
        intel-gfx@...ts.freedesktop.org, llvm@...ts.linux.dev,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, mripard@...nel.org
Subject: Re: [Intel-gfx] [PATCH v2] drm/i915/tv: avoid possible division by
 zero

On 2023/7/25 01:35, Andi Shyti wrote:
> On Tue, Jul 18, 2023 at 09:32:17AM +0800, 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 | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/i915/display/intel_tv.c b/drivers/gpu/drm/i915/display/intel_tv.c
>> index 36b479b46b60..f59553f7c132 100644
>> --- a/drivers/gpu/drm/i915/display/intel_tv.c
>> +++ b/drivers/gpu/drm/i915/display/intel_tv.c
>> @@ -988,7 +988,7 @@ 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 << !tv_mode->progressive;
> but this does not provide the same value. Try with:
>
> 	8 / (2 >> 1)
>
> and
>
> 	8 / 2 << 1
>
> They are definitely different.
>
> The real check could be:
>
> 	if (!(tv_mode->oversample >> 1))
> 		return ...
>
> But first I would check if that's actually possible.

Oh, I have a v3 patch, like this:

-       mode->clock = clock / (tv_mode->oversample >> 
!tv_mode->progressive);
+       mode->clock = clock;
+       if (tv_mode->oversample >> !tv_mode->progressive)
+               mode->clock /= tv_mode->oversample >> 1;

But I'm not sure does it need to print some error messages or do some 
things  when
"tv_mode->oversample << !tv_mode->progressive" is zero?
If all right , I will send this v3 patch.

Su Hui

> Andi