Re: [PATCH] drm/i915: protect against division by zero.

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

Op 23-08-18 om 02:51 schreef Rodrigo Vivi:
> Static analyzer tools thinks it is possible to have a division by zero
> here.
>
> I don't believe we would really reach this path without any crtc enabled,
> but may be good to protect against some unexpected path or behavior.
>
> Fixes: cf1f697acb04 ("drm/i915/skl: distribute DDB based on panel resolution")
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Mahesh Kumar <mahesh1.kumar@intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_pm.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> index d99e5fabe93c..fac6e159a640 100644
> --- a/drivers/gpu/drm/i915/intel_pm.c
> +++ b/drivers/gpu/drm/i915/intel_pm.c
> @@ -3878,6 +3878,12 @@ skl_ddb_get_pipe_allocation_limits(struct drm_device *dev,
>  			pipe_width = hdisplay;
>  	}
>  
> +	if (WARN_ON(total_width == 0)) {
> +		alloc->start = 0;
> +		alloc->end = 0;
> +		return;
> +	}
> +
>  	alloc->start = ddb_size * width_before_pipe / total_width;
>  	alloc->end = ddb_size * (width_before_pipe + pipe_width) / total_width;
>  }

It's actually quite impossible.. only way I can imagine if hw is already borked.

It must be the case that we will set cstate->active, which implies cstate->enable,
which implies a valid mode is set at least on the current crtc.

I don't think static analysis tools could deduce it, but definitely a false positive.

Also too generic commit message oneliner?

~Maarten

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