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

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Date:   Mon, 17 Jul 2023 16:52:51 +0200
From:   Andrzej Hajda <andrzej.hajda@...el.com>
To:     Su Hui <suhui@...china.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 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;

Btw in both cases there is assumption tv_mode->oversample != 0, I guess 
it is true.

Regards
Andrzej

>   
>   	/*
>   	 * tv_mode horizontal timings: