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

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Date:   Tue, 18 Jul 2023 13:28:38 +0200
From:   Andrzej Hajda <andrzej.hajda@...el.com>
To:     Su Hui <suhui@...china.com>,
        Dan Carpenter <dan.carpenter@...aro.org>
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 18.07.2023 12:10, Su Hui wrote:
> 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?

Speed gain here is irrelevant here, and disputable.

One thing which could be problematic is that we could loose the least 
significant bit in mode->clock,
in case non-progressive, but I am not sure if it really matters, as 
mode->clock is not precise value anyway.
Alternatively we could 1st shift, then divide, but in this case overflow 
can occur, at least in theory - I suspect there are no such big clocks 
(in kHz).

Finally I would agree with Dan, readability is better with conditional.

Regards
Andrzej

>
> Su Hui
>
>> regards,
>> dan carpenter