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Message-ID: <20200416005409.GR4758@pendragon.ideasonboard.com>
Date: Thu, 16 Apr 2020 03:54:09 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Doug Anderson <dianders@...omium.org>
Cc: Stephen Boyd <swboyd@...omium.org>,
Andrzej Hajda <a.hajda@...sung.com>,
David Airlie <airlied@...ux.ie>,
Daniel Vetter <daniel@...ll.ch>,
Neil Armstrong <narmstrong@...libre.com>,
Rob Herring <robh+dt@...nel.org>,
Sandeep Panda <spanda@...eaurora.org>,
Jonas Karlman <jonas@...boo.se>,
Bjorn Andersson <bjorn.andersson@...aro.org>,
"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS"
<devicetree@...r.kernel.org>,
Jeffrey Hugo <jeffrey.l.hugo@...il.com>,
Jernej Skrabec <jernej.skrabec@...l.net>,
linux-arm-msm <linux-arm-msm@...r.kernel.org>,
Rob Clark <robdclark@...omium.org>,
dri-devel <dri-devel@...ts.freedesktop.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] dt-bindings: drm/bridge: ti-sn65dsi86: Add hpd-gpios
to the bindings
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@...omium.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
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