[PATCH] drm/i915/display: Avoid divide by zero

[Juha-Pekka Heikkilä]

On Thu, Aug 21, 2025 at 4:05 PM Ville Syrjälä
<ville.syrjala at linux.intel.com> wrote:
>
> On Wed, Aug 20, 2025 at 04:29:13PM +0300, Juha-Pekka Heikkila wrote:
> > skl_crtc_allocate_plane_ddb allow iter.data_rate to be zero
> > which could cause divide by zero in skl_allocate_plane_ddb,
> > check against that.
> >
> > Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila at gmail.com>
> > ---
> >  drivers/gpu/drm/i915/display/skl_watermark.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/skl_watermark.c b/drivers/gpu/drm/i915/display/skl_watermark.c
> > index def5150231a4..403783504ab3 100644
> > --- a/drivers/gpu/drm/i915/display/skl_watermark.c
> > +++ b/drivers/gpu/drm/i915/display/skl_watermark.c
> > @@ -1388,7 +1388,7 @@ skl_allocate_plane_ddb(struct skl_plane_ddb_iter *iter,
> >  {
> >       u16 size, extra = 0;
> >
> > -     if (data_rate) {
> > +     if (data_rate && iter->data_rate) {
>
> I don't think it should be possible to have iter_data->data_rate==0
> and data_rate!=0, with iter->data_rate being the sum of all the
> planes' data_rates.
>

But I see there in calling function skl_crtc_allocate_plane_ddb(..)
just before arriving here, check you've left there few years ago that
says

/* avoid the WARN later when we don't allocate any extra DDB */
if (iter.data_rate == 0)
iter.size = 0;

hence I had idea to add this check also

> >               extra = min_t(u16, iter->size,
> >                             DIV64_U64_ROUND_UP(iter->size * data_rate,
> >                                                iter->data_rate));
> > --
> > 2.43.0
>
> --
> Ville Syrjälä
> Intel