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

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Date:   Tue, 18 Jul 2023 18:10:44 +0800
From:   Su Hui <suhui@...china.com>
To:     Dan Carpenter <dan.carpenter@...aro.org>,
        Andrzej Hajda <andrzej.hajda@...el.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,
        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 2023/7/18 13:39, Dan Carpenter wrote:
> On Mon, Jul 17, 2023 at 04:52:51PM +0200, 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;
> This is nice.

mode->clock = clock / tv_mode->oversample << !tv_mode->progressive;

But I think this one is much better,  it has less code and run faster.
Should I resend v3 to add some explanation or follow Dan's advice?

Su Hui

> regards,
> dan carpenter