linux-kernel - Re: [PATCH v2 3/4] drm/bridge: ti-sn65dsi86: Read EDID blob over DDC

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Message-ID: <CAD=FV=VKTS7G9a3x8iHg=eWRFtrcwKBdwbdtynmHhV4KPCnDKQ@mail.gmail.com>
Date:   Mon, 2 Nov 2020 08:06:14 -0800
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>,
        LKML <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Sean Paul <seanpaul@...omium.org>
Subject: Re: [PATCH v2 3/4] drm/bridge: ti-sn65dsi86: Read EDID blob over DDC

Hi,

On Sun, Nov 1, 2020 at 11:21 AM Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> Hi Stephen,
>
> Thank you for the patch.
>
> On Thu, Oct 29, 2020 at 06:17:37PM -0700, Stephen Boyd wrote:
> > Use the DDC connection to read the EDID from the eDP panel instead of
> > relying on the panel to tell us the modes.
> >
> > Reviewed-by: Douglas Anderson <dianders@...omium.org>
> > Cc: Laurent Pinchart <Laurent.pinchart@...asonboard.com>
> > Cc: Jonas Karlman <jonas@...boo.se>
> > Cc: Jernej Skrabec <jernej.skrabec@...l.net>
> > Cc: Sean Paul <seanpaul@...omium.org>
> > Signed-off-by: Stephen Boyd <swboyd@...omium.org>
> > ---
> >  drivers/gpu/drm/bridge/ti-sn65dsi86.c | 20 ++++++++++++++++++++
> >  1 file changed, 20 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > index c77f46a21aae..f86934fd6cc8 100644
> > --- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > +++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > @@ -119,6 +119,7 @@
> >   * @debugfs:      Used for managing our debugfs.
> >   * @host_node:    Remote DSI node.
> >   * @dsi:          Our MIPI DSI source.
> > + * @edid:         Detected EDID of eDP panel.
> >   * @refclk:       Our reference clock.
> >   * @panel:        Our panel.
> >   * @enable_gpio:  The GPIO we toggle to enable the bridge.
> > @@ -144,6 +145,7 @@ struct ti_sn_bridge {
> >       struct drm_bridge               bridge;
> >       struct drm_connector            connector;
> >       struct dentry                   *debugfs;
> > +     struct edid                     *edid;
> >       struct device_node              *host_node;
> >       struct mipi_dsi_device          *dsi;
> >       struct clk                      *refclk;
> > @@ -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);
> > +     }
>
> Do we need to cache the EDID ? It seems like something that should be
> done by the DRM core (well, caching modes in that case), not by
> individual bridge drivers.

I can take the blame for the fact that it does caching, since I
requested it in early reviews.  In general boot speed is pretty
important to me and each read of the EDID take 20 ms.  There are
definitely several calls to get the EDID during a normal bootup.
Stephen did a little more digging into exactly what was causing all
these calls and can chime in, but in general until we can eliminate
the extra calls it seems like it'd be nice to keep the caching?  This
bridge chip is intended for use for eDP for internal panels, so there
should be no downside to caching.  If we can later optimize the DRM
core, we can fix this and a pre-existing driver that does the same
type of caching (analogix-anx6345.c) at the same time?

-Doug