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

[Rodrigo Vivi <rodrigo.vivi@intel.com>]

On Thu, Aug 23, 2018 at 07:11:16AM +0200, Maarten Lankhorst wrote:
> 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.

yeap... that's what I thought... but I decided to check just in case I missed something.

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

I'll mart that so...

> 
> Also too generic commit message oneliner?

it just reflects the amount of confidence I had on the need of this ;)

just dropping it then.

thanks for the confirmation

> 
> ~Maarten
> 
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