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    SubjectRe: [PATCH] drm: mxsfb: Set proper default bus format when using a bridge
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    Am Dienstag, dem 28.09.2021 um 11:19 +0200 schrieb Guido Günther:
    > Hi,
    > On Tue, Sep 28, 2021 at 11:08:58AM +0200, Marek Vasut wrote:
    > > On 9/28/21 10:55 AM, Guido Günther wrote:
    > > > If a bridge doesn't do any bus format handling MEDIA_BUS_FMT_FIXED is
    > > > returned. Fallback to a reasonable default (MEDIA_BUS_FMT_RGB888_1X24) in
    > > > that case.
    > > >
    > > > This unbreaks e.g. using mxsfb with the nwl bridge and mipi panels.
    > > >
    > > > Fixes: b776b0f00f24 ("drm: mxsfb: Use bus_format from the nearest bridge if present")
    > > >
    > > > Signed-off-by: Guido Günther <agx@sigxcpu.org>
    > > > ---
    > > >
    > > > I'll look at what needs to be done in nwl separately but this also
    > > > unbreaks other bridge seupts that don't to format negotiation yet.
    > > >
    > > > drivers/gpu/drm/mxsfb/mxsfb_kms.c | 2 ++
    > > > 1 file changed, 2 insertions(+)
    > > >
    > > > diff --git a/drivers/gpu/drm/mxsfb/mxsfb_kms.c b/drivers/gpu/drm/mxsfb/mxsfb_kms.c
    > > > index af6c620adf6e..4ef94cf686b0 100644
    > > > --- a/drivers/gpu/drm/mxsfb/mxsfb_kms.c
    > > > +++ b/drivers/gpu/drm/mxsfb/mxsfb_kms.c
    > > > @@ -369,6 +369,8 @@ static void mxsfb_crtc_atomic_enable(struct drm_crtc *crtc,
    > > > drm_atomic_get_new_bridge_state(state,
    > > > mxsfb->bridge);
    > > > bus_format = bridge_state->input_bus_cfg.format;
    > > > + if (bus_format == MEDIA_BUS_FMT_FIXED)
    > > > + bus_format = MEDIA_BUS_FMT_RGB888_1X24;
    > >
    > > Shouldn't the NWL bridge return the correct format ?
    >
    > Yes it should and I'll send a separate patch for that but we currently
    > don't do anything meaningful at all if the bridge doesn't do format
    > negotiation and then fail setup in mxsfb_set_formats().
    >
    > I think we should at least preserve the status quo (as we do with the
    > non bridge case in b776b0f00f24 too).
    >
    > We could have a warning to spot drivers that don't do that yet and hence
    > the generic code returns MEDIA_BUS_FMT_FIXED.
    >
    That sounds sensible. Using a default format if we don't know what to
    do is going to be a unpleasant surprise for those with a display
    pipeline that doesn't work with the default format. So please add a
    dev_warn when we are doing this fallback.

    Also I would argue that the NWL fix is the patch that should go in the
    stable tree. This one should only be a additional safety net, so I
    would drop the Fixes tag.

    Regards,
    Lucas

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