linux-kernel - Re: [PATCH 6/7] drm/vc4: dpi: Change the default DPI format to being 18bpp, not 24.

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Message-ID: <CAPY8ntAiX6WJ86wGUgMvTxhehG8OgoS66AojvWYukADciM5X5w@mail.gmail.com>
Date:   Mon, 17 Oct 2022 18:03:03 +0100
From:   Dave Stevenson <dave.stevenson@...pberrypi.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Maxime Ripard <maxime@...no.tech>,
        Maxime Ripard <mripard@...nel.org>,
        Eric Anholt <eric@...olt.net>, Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>, Rob Herring <robh@...nel.org>,
        Emma Anholt <emma@...olt.net>, dri-devel@...ts.freedesktop.org,
        Joerg Quinten <aBUGSworstnightmare@...il.com>,
        Chris Morgan <macromorgan@...mail.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/7] drm/vc4: dpi: Change the default DPI format to being
 18bpp, not 24.

Hi Laurent

On Sat, 15 Oct 2022 at 18:14, Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> Hi Maxime (and Dave),
>
> Thank you for the patch.
>
> On Thu, Oct 13, 2022 at 11:56:50AM +0200, Maxime Ripard wrote:
> > From: Dave Stevenson <dave.stevenson@...pberrypi.com>
> >
> > DPI hasn't really been used up until now, so the default has
> > been meaningless.
> > In theory we should be able to pass the desired format for the
> > adjacent bridge chip through, but framework seems to be missing
> > for that.
>
> Doesn't the bridge infrastructure allow that ? Or maybe this commit
> message was written a while ago, before it was possible ?

Infrastructure may do, but it isn't always implemented.
Rightly or wrongly, the driver is currently finding the associated
connector, as commented as [1].

The main chain I'm looking is:
vc4_dpi -> dumb-vga-dac -> vga-connector
Unless I'm missing something, nothing in that chain defines the format
for the output. vga-connector (display-connector driver) tries to ask
the previous bridge for formats via atomic_get_output_bus_fmts and
atomic_get_input_fmts, but dumb-vga-dac (simple-bridge) doesn't
implement them.
So even if we tried following the chain it dies due to , and currently
the connector has no bus_formats defined in display_info, hence we end
up needing a default.
(Why do I get deja vu with panel-dpi and bus-format discussions?!)

Panels are fine as they do generally have a bus_format defined in display_info.

[1] https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/vc4/vc4_dpi.c#L139

> In any case, it would be nice to use the bus format exposed by the next
> bridge in the chain, but that can be done in a subsequent step. The new
> default seems reasonable.

I'll add having a look at atomic_get_output_bus_fmts and
atomic_get_input_fmts to my list of tasks again.

  Dave

> Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
>
> > As the main device to use DPI is the VGA666 or Adafruit Kippah,
> > both of which use RGB666, change the default to being RGB666 instead
> > of RGB888.
> >
> > Signed-off-by: Dave Stevenson <dave.stevenson@...pberrypi.com>
> > Signed-off-by: Maxime Ripard <maxime@...no.tech>
> > ---
> >  drivers/gpu/drm/vc4/vc4_dpi.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/vc4/vc4_dpi.c b/drivers/gpu/drm/vc4/vc4_dpi.c
> > index ecbe4cd87036..fdae02760b6d 100644
> > --- a/drivers/gpu/drm/vc4/vc4_dpi.c
> > +++ b/drivers/gpu/drm/vc4/vc4_dpi.c
> > @@ -150,8 +150,8 @@ static void vc4_dpi_encoder_enable(struct drm_encoder *encoder)
> >       }
> >       drm_connector_list_iter_end(&conn_iter);
> >
> > -     /* Default to 24bit if no connector or format found. */
> > -     dpi_c |= VC4_SET_FIELD(DPI_FORMAT_24BIT_888_RGB, DPI_FORMAT);
> > +     /* Default to 18bit if no connector or format found. */
> > +     dpi_c |= VC4_SET_FIELD(DPI_FORMAT_18BIT_666_RGB_1, DPI_FORMAT);
> >
> >       if (connector) {
> >               if (connector->display_info.num_bus_formats) {
> >
>
> --
> Regards,
>
> Laurent Pinchart