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

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Date:   Mon, 24 Jul 2023 19:35:27 +0200
From:   Andi Shyti <andi.shyti@...ux.intel.com>
To:     Su Hui <suhui@...china.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 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.

Andi