linux-kernel - Re: [PATCH v2 0/4] drm/bridge: ti-sn65dsi86: Support EDID reading

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Date:   Tue, 23 Mar 2021 02:00:23 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Sam Ravnborg <sam@...nborg.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Jonas Karlman <jonas@...boo.se>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        Douglas Anderson <dianders@...omium.org>,
        Sean Paul <seanpaul@...omium.org>
Subject: Re: [PATCH v2 0/4] drm/bridge: ti-sn65dsi86: Support EDID reading

Hi Stephen,

On Mon, Nov 02, 2020 at 05:15:24PM -0800, Stephen Boyd wrote:
> Quoting Sam Ravnborg (2020-11-01 09:37:41)
> > Hi Stephen.
> > 
> > On Thu, Oct 29, 2020 at 06:17:34PM -0700, Stephen Boyd wrote:
> > > This patch series cleans up the DDC code a little bit so that
> > > it is more efficient time wise and supports grabbing the EDID
> > > of the eDP panel over the aux channel. I timed this on a board
> > > I have on my desk and it takes about 20ms to grab the EDID out
> > > of the panel and make sure it is valid.
> > > 
> > > The first two patches seem less controversial so I stuck them at
> > > the beginning. The third patch does the EDID reading and caches
> > > it so we don't have to keep grabbing it over and over again. And
> > > finally the last patch updates the reply field so that short
> > > reads and nacks over the channel are reflected properly instead of
> > > treating them as some sort of error that can't be discerned.
> > > 
> > > Stephen Boyd (4):
> > >   drm/bridge: ti-sn65dsi86: Combine register accesses in
> > >     ti_sn_aux_transfer()
> > >   drm/bridge: ti-sn65dsi86: Make polling a busy loop
> > >   drm/bridge: ti-sn65dsi86: Read EDID blob over DDC
> > >   drm/bridge: ti-sn65dsi86: Update reply on aux failures
> > 
> > Series looks good. You can add my a-b on the full series.
> > Acked-by: Sam Ravnborg <sam@...nborg.org>
> > 
> > I can apply after Douglas have had a look at the patches he did not r-b
> > yet.
> > 
> > Any chance we can convince you to prepare this bridge driver for use in
> > a chained bridge setup where the connector is created by the display
> > driver and uses drm_bridge_funcs?
> > 
> > First step wuld be to introduce the use of a panel_bridge.
> > Then add get_edid to drm_bridge_funcs and maybe more helpers.
> > 
> > Then natural final step would be to move connector creation to the
> > display driver - see how other uses drm_bridge_connector_init() to do so
> > - it is relatively simple.
> > 
> > Should be doable - and reach out if you need some help.
> 
> I started to look at this and got stuck at ti_sn_bridge_get_bpp(). Where
> can I get the details of the bpc for the downstream bridge or panel? Is
> there some function that can tell this bridge what the bpc is for the
> attached connector?

I've posted a patch series to convert to DRM_BRIDGE_ATTACH_NO_CONNECTOR
yesterday (and have CC'ed you), but I've overlooked this particular
problem :-S

You can't get the connector in the .enable() operation, but you can get
it in .atomic_enable(), with
drm_atomic_get_new_connector_for_encoder(). This being said, it's
probably not the right option.

What matters here isn't the bpc for the connector, but the format
expected by the next bridge in the chain. drm_bridge_funcs has two
operations, .atomic_get_output_bus_fmts() and
.atomic_get_input_bus_fmts(), to negotiate that format along a chain of
bridges. The panel bridge driver (drivers/gpu/drm/bridge/panel.c)
doesn't implement those operations, and neither does
display-connector.c, so that may be what we should start with.

> I see that td_mode_valid() in
> drivers/gpu/drm/bridge/tc358775.c stores away the bpc from the incoming
> drm_display_info pointer but I'm not sure that is correct because can't
> that be called for various and not necessarily the one we're using?

You're right, .mode_valid() shouldn't do that.

-- 
Regards,

Laurent Pinchart