linux-kernel - Re: [PATCH] drm: mxsfb: Set proper default bus format when using a bridge

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Date:   Tue, 28 Sep 2021 11:27:49 +0200
From:   Lucas Stach <l.stach@...gutronix.de>
To:     Guido Günther <agx@...xcpu.org>,
        Marek Vasut <marex@...x.de>
Cc:     Stefan Agner <stefan@...er.ch>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Sam Ravnborg <sam@...nborg.org>,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH] drm: mxsfb: Set proper default bus format when using a
 bridge

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@...xcpu.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