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Date:   Thu, 16 Apr 2020 03:54:09 +0300
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Doug Anderson <dianders@chromium.org>
Cc:     Stephen Boyd <swboyd@chromium.org>,
        Andrzej Hajda <a.hajda@samsung.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Neil Armstrong <narmstrong@baylibre.com>,
        Rob Herring <robh+dt@kernel.org>,
        Sandeep Panda <spanda@codeaurora.org>,
        Jonas Karlman <jonas@kwiboo.se>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        Jeffrey Hugo <jeffrey.l.hugo@gmail.com>,
        Jernej Skrabec <jernej.skrabec@siol.net>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        Rob Clark <robdclark@chromium.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/3] dt-bindings: drm/bridge: ti-sn65dsi86: Add hpd-gpios
 to the bindings
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Hi Doug,

On Wed, Apr 15, 2020 at 04:49:00PM -0700, Doug Anderson wrote:
> On Wed, Apr 15, 2020 at 1:33 PM Laurent Pinchart wrote:
> > On Wed, Apr 15, 2020 at 12:53:02PM -0700, Stephen Boyd wrote:
> > > Quoting Douglas Anderson (2020-04-15 08:48:40)
> > > > Allow people to specify to use a GPIO for hot-plug-detect.  Add an
> > > > example.
> > > >
> > > > NOTE: The current patch adding support for hpd-gpios to the Linux
> > > > driver for hpd-gpios only adds enough support to the driver so that
> > > > the bridge can use one of its own GPIOs.  The bindings, however, are
> > > > written generically.
> > > >
> > > > Signed-off-by: Douglas Anderson <dianders@chromium.org>
> > > > ---
> > > >
> > > >  .../bindings/display/bridge/ti,sn65dsi86.yaml          | 10 +++++++++-
> > > >  1 file changed, 9 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> > > > index 8cacc6db33a9..554bfd003000 100644
> > > > --- a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> > > > +++ b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> > > > @@ -60,6 +60,10 @@ properties:
> > > >      const: 1
> > > >      description: See ../../pwm/pwm.yaml for description of the cell formats.
> > > >
> > > > +  hpd-gpios:
> > > > +    maxItems: 1
> > > > +    description: If present use the given GPIO for hot-plug-detect.
> > >
> > > Shouldn't this go in the panel node? And the panel driver should get the
> > > gpio and poll it after powering up the panel? Presumably that's why we
> > > have the no-hpd property in the simple panel binding vs. putting it here
> > > in the bridge.
> >
> > Same question really, I think this belongs to the panel (or connector)
> > node indeed.
> 
> Hrm.
> 
> To me "no-hpd" feels OK in the panel because the lack of a connection
> is somewhat symmetric.  Thus it's OK to say either "HPD isn't hooked
> up to the panel in this system" or "HPD isn't hooked up to the bridge
> in this system" and both express the same thing (AKA that there is no
> HPD connection between the bridge and the panel).  In the case of
> "no-hpd" it's more convenient to express it on the panel side because
> the panel driver is the one whose behavior has to change if HPD isn't
> hooked up.  The panel datasheet is the one that says how long of a
> delay we need if HPD isn't hooked up.
> 
> ...but when you're talking about where the bridge driver should look
> to find the HPD signal that it needs, that really feels like it should
> be described as part of the bridge.  Specifically imagine we were
> using our bridge for DP, not for eDP.  In that case simple-panel
> wouldn't be involved because we could get any type of display plugged
> in.  Thus it couldn't go in the panel node.  Here it feels clearer
> that hpd-gpio needs to be a property of the bridge driver.

If you were using it for DP, you would need a DT node for the DP
connector (with bindings to be added to
Documentation/devicetree/bindings/display/connector/, similar to the
ones we already have for other connectors). That DT node should
reference the HPD pin GPIO. The bridge driver for the connector
(drivers/gpu/drm/bridge/display-connector.c) would then handle HPD. The
good news is that it already does :-)

> Looking at other usages of "hpd-gpio" in the kernel, it seems like the
> usage I'm proposing is also common.  Grepping for "hpd-gpios" shows
> numerous examples of "hpd-gpios" being defined at the display
> controller level and (effectively) I believe the bridge is at the
> equivalent level.

Bridge drivers should only implement support for features available from
the corresponding hardware. If an HPD signal is connected to a dedicated
pin of the bridge, and the bridge can generate an interrupt and expose
the HPD status through I2C, then it should implement HPD-related
operations. If the HPD pin from the connector is hooked up to a GPIO of
the SoC, it should be handled by the connector bridge driver.

-- 
Regards,

Laurent Pinchart