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

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Message-ID: <160436612483.884498.883110130131457033@swboyd.mtv.corp.google.com>
Date:   Mon, 02 Nov 2020 17:15:24 -0800
From:   Stephen Boyd <swboyd@...omium.org>
To:     Sam Ravnborg <sam@...nborg.org>
Cc:     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>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v2 0/4] drm/bridge: ti-sn65dsi86: Support EDID reading

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 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?