Re: [PATCH v3 1/9] drm/mediatek: dp: Cache EDID for eDP panel

[Matthias Brugger]

On 12/04/2023 10:06, AngeloGioacchino Del Regno wrote:
> Il 12/04/23 09:08, Matthias Brugger ha scritto:
> > On 04/04/2023 12:47, AngeloGioacchino Del Regno wrote:
> > > Since eDP panels are not removable it is safe to cache the EDID:
> > > this will avoid a relatively long read transaction at every PM
> > > resume that is unnecessary only in the "special" case of eDP,
> > > hence speeding it up a little, as from now on, as resume operation,
> > > we will perform only link training.
> > >
> > > Signed-off-by: AngeloGioacchino Del Regno 
> > > <angelogioacchino.delregno@xxxxxxxxxxxxx>
> > > ---
> > >   drivers/gpu/drm/mediatek/mtk_dp.c | 11 ++++++++++-
> > >   1 file changed, 10 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/gpu/drm/mediatek/mtk_dp.c 
> > > b/drivers/gpu/drm/mediatek/mtk_dp.c
> > > index 1f94fcc144d3..84f82cc68672 100644
> > > --- a/drivers/gpu/drm/mediatek/mtk_dp.c
> > > +++ b/drivers/gpu/drm/mediatek/mtk_dp.c
> > > @@ -118,6 +118,7 @@ struct mtk_dp {
> > >       const struct mtk_dp_data *data;
> > >       struct mtk_dp_info info;
> > >       struct mtk_dp_train_info train_info;
> > > +    struct edid *edid;
> > >       struct platform_device *phy_dev;
> > >       struct phy *phy;
> > > @@ -1993,7 +1994,11 @@ static struct edid *mtk_dp_get_edid(struct drm_bridge 
> > > *bridge,
> > >           usleep_range(2000, 5000);
> > >       }
> > > -    new_edid = drm_get_edid(connector, &mtk_dp->aux.ddc);
> > > +    /* eDP panels aren't removable, so we can return a cached EDID. */
> > > +    if (mtk_dp->edid && mtk_dp->bridge.type == DRM_MODE_CONNECTOR_eDP)

Maybe better like this:
if (mtk_dp->bridge.type == DRM_MODE_CONNECTOR_eDP && mtk_dp->edid)

To in sync with the if statement below. Anyway we are only concerned if it's an 
eDP so check that first (and hope the compiler will do so as well ;)

With that:
Reviewed-by: Matthias Brugger <matthias.bgg@xxxxxxxxx>

> > > +        new_edid = drm_edid_duplicate(mtk_dp->edid);
> > > +    else
> > > +        new_edid = drm_get_edid(connector, &mtk_dp->aux.ddc);
> >
> > Maybe it would make sense to add a macro for the check of mtk_dp->bridge.type 
> > == DRM_MODE_CONNECTOR_eDP
> > it would make the code more readable.
>
> I had the same idea... but then avoided that because in most (if not all?) of the
> DRM drivers (at least, the one I've read) this check is always open coded, so I
> wrote it like that for consistency and nothing else.
>
> I have no strong opinions on that though!

I think the only reasonable solution would be a macro like:
DRM_CONNECTOR_MODE_IS(mtk_dp->bridge.type, eDP) which in the end is longer then 
open-code it, so probably just leave it as it is.

> > >       /*
> > >        * Parse capability here to let atomic_get_input_bus_fmts and
> > > @@ -2022,6 +2027,10 @@ static struct edid *mtk_dp_get_edid(struct drm_bridge 
> > > *bridge,
> > >           drm_atomic_bridge_chain_post_disable(bridge, connector->state->state);
> > >       }
> > > +    /* If this is an eDP panel and the read EDID is good, cache it for later */
> > > +    if (mtk_dp->bridge.type == DRM_MODE_CONNECTOR_eDP && !mtk_dp->edid && 
> > > new_edid)
> > > +        mtk_dp->edid = drm_edid_duplicate(new_edid);
> > > +
> >
> > How about putting this in an else if branch of mtk_dp_parse_capabilities. At 
> > least we could get rid of the check regarding if new_edid != NULL.
> >
> > I was thinking on how to put both if statements in one block, but I think the 
> > problem is, that we would leak memory if the capability parsing failes due to 
> > the call to drm_edid_duplicate(). Correct?
>
> Correct. The only other "good" place would be in the `if (new_edid)` conditional,
> but that wouldn't be as readable as it is right now...
>
> Cheers,
> Angelo