Re: [PATCH v6 3/8] drm/bridge: mhdp8546: Add minimal format negotiation

[Neil Armstrong]

On 15/05/2023 17:59, Aradhya Bhatia wrote:
> Hi Tomi,
>
> On 12-May-23 14:45, Tomi Valkeinen wrote:
> > On 09/05/2023 12:30, Aradhya Bhatia wrote:
> > > From: Nikhil Devshatwar <nikhil.nd@xxxxxx>
> > >
> > > With new connector model, mhdp bridge will not create the connector and
> > > SoC driver will rely on format negotiation to setup the encoder format.
> > >
> > > Support minimal format negotiations hooks in the drm_bridge_funcs.
> > > Complete format negotiation can be added based on EDID data.
> > > This patch adds the minimal required support to avoid failure
> > > after moving to new connector model.
> > >
> > > Signed-off-by: Nikhil Devshatwar <nikhil.nd@xxxxxx>
> > > Reviewed-by: Tomi Valkeinen <tomi.valkeinen@xxxxxx>
> >
> > You need to add your SoB to this and the other patches.
>
> Okay!
>
> > > ---
> > >
> > > Notes:
> > >
> > >       changes from v1:
> > >       * cosmetic fixes, commit message update.
> > >
> > >       changes from v5:
> > >       * dropped the default_bus_format variable and directly assigned
> > >         MEDIA_BUS_FMT_RGB121212_1X36 to input_fmts.
> > >
> > >    .../drm/bridge/cadence/cdns-mhdp8546-core.c   | 25 +++++++++++++++++++
> > >    1 file changed, 25 insertions(+)
> > >
> > > diff --git a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
> > > b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
> > > index f6822dfa3805..623e4235c94f 100644
> > > --- a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
> > > +++ b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
> > > @@ -2146,6 +2146,30 @@ cdns_mhdp_bridge_atomic_reset(struct drm_bridge
> > > *bridge)
> > >        return &cdns_mhdp_state->base;
> > >    }
> > >    +static u32 *cdns_mhdp_get_input_bus_fmts(struct drm_bridge *bridge,
> > > +                     struct drm_bridge_state *bridge_state,
> > > +                     struct drm_crtc_state *crtc_state,
> > > +                     struct drm_connector_state *conn_state,
> > > +                     u32 output_fmt,
> > > +                     unsigned int *num_input_fmts)
> > > +{
> > > +    u32 *input_fmts;
> > > +
> > > +    *num_input_fmts = 0;
> > > +
> > > +    if (output_fmt != MEDIA_BUS_FMT_FIXED)
> > > +        return NULL;
> >
> > The tfp410 and sii902x drivers don't have the above check. Why does mhdp
> > need it? Or the other way, why don't tfp410 and sii902x need it?
>
> I had removed this condition in order to follow status quo, from the
> ITE-66121 HDMI bridge driver.
>
> The idea would have been to drop this for MHDP as well, but I guess I
> overlooked this one.
>
> However...
>
> > I guess at the moment we always do get MEDIA_BUS_FMT_FIXED as the out
> > fmt (in all three bridge drivers), don't we?
>
> ... I tested again to ensure that the above is indeed the case. And
> ended up catching some odd behavior.
>
> It turns out that for all the HDMI bridges (TFP410, SII902X, ITE-66121),
> the format negotiation doesn't stop at output_fmt = MEDIA_BUS_FMT_FIXED.
> The {bridge}_get_input_format API gets called again with the output_fmt
> = MEDIA_BUS_FMT_RGB24_1X24.
>
> This doesn't happen with the MHDP driver. Format negotiation with MHDP
> bridge stops after one round, at output_fmt = MEDIA_BUS_FMT_FIXED.

This is because the bridge negociation logic will test with all possible
output formats from the chain, and won't stop at first working test.

If your bridge only supports a single input format, it should return the
same format whatever output_fmt is tried.

So indeed remove this test on mhdp aswell, or filter out invalid output
formats.

The MEDIA_BUS_FMT_FIXED is when there's no output format to test, so this
should be always supported.

Neil

> Regards
> Aradhya