linux-kernel - Re: [PATCH v3 3/4] drm/bridge: ti-sn65dsi86: Read EDID blob over DDC

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Message-ID: <CAD=FV=WfrZz4PsDcEeLkvsFa6p9LOo1G-3e00NzoVLZ713xNpg@mail.gmail.com>
Date:   Fri, 7 May 2021 14:32:07 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Stephen Boyd <swboyd@...omium.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Sam Ravnborg <sam@...nborg.org>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Jonas Karlman <jonas@...boo.se>,
        LKML <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Sean Paul <seanpaul@...omium.org>
Subject: Re: [PATCH v3 3/4] drm/bridge: ti-sn65dsi86: Read EDID blob over DDC

Hi,

On Tue, Mar 23, 2021 at 12:53 PM Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> Hi Doug,
>
> On Tue, Mar 23, 2021 at 12:07:27PM -0700, Doug Anderson wrote:
> > On Mon, Mar 22, 2021 at 8:17 PM Stephen Boyd <swboyd@...omium.org> wrote:
> > >
> > > Quoting Laurent Pinchart (2021-03-17 17:20:43)
> > > > Hi Stephen,
> > > >
> > > > Reviving a bit of an old thread, for a question.
> > > >
> > > > On Mon, Nov 02, 2020 at 10:11:43AM -0800, Stephen Boyd wrote:
> > > > > @@ -265,6 +267,23 @@ connector_to_ti_sn_bridge(struct drm_connector *connector)
> > > > >  static int ti_sn_bridge_connector_get_modes(struct drm_connector *connector)
> > > > >  {
> > > > >       struct ti_sn_bridge *pdata = connector_to_ti_sn_bridge(connector);
> > > > > +     struct edid *edid = pdata->edid;
> > > > > +     int num, ret;
> > > > > +
> > > > > +     if (!edid) {
> > > > > +             pm_runtime_get_sync(pdata->dev);
> > > > > +             edid = pdata->edid = drm_get_edid(connector, &pdata->aux.ddc);
> > > > > +             pm_runtime_put(pdata->dev);
> > > >
> > > > Is there any specific reason to use the indirect access method, compared
> > > > to the direct method that translates access to an I2C ancillary address
> > > > to an I2C-over-AUX transaction (see page 20 of SLLSEH2B) ? The direct
> > > > method seems it would be more efficient.
> > >
> > > No I don't think it matters. I was just using the existing support code
> > > that Sean wrote instead of digging into the details. Maybe Sean ran into
> > > something earlier and abandoned that approach?
> >
> > From reading the docs, it sounds as if there _could_ be a reason to
> > use the indirect method. Specifically if the i2c host that the bridge
> > is on doesn't support clock stretching then the direct method wouldn't
> > work according to the docs. Is that something that we'd have to
> > reasonably worry about?
>
> I'm not sure. I'm going through BSP code that uses the direct method,
> and I was wondering if it was just an implementation detail. Once I get
> the display working on this board, I'll try to find time to compare the
> two methods, to see if there's a significatant performance improvement
> from the direct method. If there isn't, I won't bother.

To follow-up here:

We'd actually been using the "direct" method in the BIOS (coreboot)
and just found a problem. We're now switching coreboot to the
"indirect" mode. Specifically we found that, at least on one panel,
the last byte of the extension block (which should have been a CRC)
was coming back as 0 when using the "direct" mode. See:

https://review.coreboot.org/c/coreboot/+/52959

In addition I was thinking about how to use "direct" mode (ignoring
the above problem) and realized that handling the power sequencing at
the right time would be hard. Maybe not a problem for you since your
bridge is always powered, but I wouldn't know how to model this in
general. Specifically if you want to talk over the i2c bus to the
panel you've got to power the bridge but I don't think the bridge gets
called in the normal code paths.

-Doug