linux-kernel - Re: [PATCH 2/4] dt-bindings: Document the Raspberry Pi Touchscreen nodes.

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Message-ID: <3768334.nZM7df9y4L@avalon>
Date:   Tue, 16 May 2017 19:54:30 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Eric Anholt <eric@...olt.net>
Cc:     Rob Herring <robh+dt@...nel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Archit Taneja <architt@...eaurora.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] dt-bindings: Document the Raspberry Pi Touchscreen nodes.

Hi Eric,

On Tuesday 16 May 2017 09:47:49 Eric Anholt wrote:
> Rob Herring <robh+dt@...nel.org> writes:
> > On Mon, May 15, 2017 at 7:03 PM, Eric Anholt <eric@...olt.net> wrote:
> >> Laurent Pinchart <laurent.pinchart@...asonboard.com> writes:
> >>> Hi Eric,
> >>> 
> >>> Thank you for the patch.
> >>> 
> >>> On Thursday 11 May 2017 16:56:23 Eric Anholt wrote:
> >>>> The Raspberry Pi 7" Touchscreen is a DPI touchscreen panel with
> >>>> DSI->DPI bridge and touchscreen controller integrated, that connects
> >>>> to the Raspberry Pi through its 15-pin "DSI" connector (some lines are
> >>>> DSI, some lines are I2C).
> >>>> 
> >>>> This device is represented in the DT as three nodes (DSI device, I2C
> >>>> device, panel).  Input will be left to a separate binding later, as it
> >>>> will be a basic I2C client device.
> >>>> 
> >>>> Signed-off-by: Eric Anholt <eric@...olt.net>
> >>>> ---
> >>>> 
> >>>>  .../raspberrypi,7inch-touchscreen-bridge.txt       | 68 ++++++++++++++
> >>>>  .../panel/raspberrypi,7inch-touchscreen-panel.txt  |  7 +++
> >>>>  2 files changed, 75 insertions(+)
> >>>>  create mode 100644
> >>>> 
> >>>> Documentation/devicetree/bindings/display/bridge/raspberrypi,7inch-touc
> >>>> hscreen-bridge.txt create mode 100644
> >>>> Documentation/devicetree/bindings/display/panel/raspberrypi,7inch-touch
> >>>> screen-panel.txt
> >>>> 
> >>>> diff --git
> >>>> a/Documentation/devicetree/bindings/display/bridge/raspberrypi,7inch-to
> >>>> uchscreen-bridge.txt
> >>>> b/Documentation/devicetree/bindings/display/bridge/raspberrypi,7inch-to
> >>>> uchscreen-bridge.txt new file mode 100644
> >>>> index 000000000000..a5669beaf68f
> >>>> --- /dev/null
> >>>> +++
> >>>> b/Documentation/devicetree/bindings/display/bridge/raspberrypi,7inch-to
> >>>> uchscreen-bridge.txt
> >>>> @@ -0,0 +1,68 @@
> >>>> +Official 7" (800x480) Raspberry Pi touchscreen panel's bridge.
> >>>> +
> >>>> +This DSI panel contains:
> >>>> +
> >>>> +- TC358762 DSI->DPI bridge
> >>>> +- Atmel microcontroller on I2C for power sequencing the DSI bridge and
> >>>> +  controlling backlight
> >>>> +- Touchscreen controller on I2C for touch input
> >>>> +
> >>>> +and this covers the TC358762 bridge and Atmel microcontroller, while
> >>>> +../panel/raspberrypi,7inch-touchscreen-panel.txt covers the panel.
> >>> 
> >>> The TC358762 is a standalone bridge that doesn't depend on the ATTiny
> >>> microcontroller used by the RPI. As it's usable standalone, I believe
> >>> this binding should be split in two.
> >> 
> >> Do you have a plan for how I would implement a driver on top of that
> >> binding change, though?  Note that we don't program the Toshiba
> >> directly, we only send commands to the Atmel.
> > 
> > I agree. If it is a black box and the interface to the host is defined
> > by the Atmel uC firmware, then that's what the DT should describe.
> > Perhaps a diagram here or pointer to one would help and remove
> > mentioning what kind of bridge chip it is.
> 
> It's a *very* black box.  I have some non-public schematics that don't
> even say what panel is involved, and no documentation of the uc
> interface.  The driver code is just replicating the firmware's
> programming sequence.
> 
> I would certainly love to be building a generic TC358762 driver, which
> would be a lot more satisfying.  I just don't think it's doable for this
> panel.  Given that, what do I need to do to the DT?  Should I just drop
> mention of the Toshiba and talk about this being a bridge with a custom
> microcontroller firmware?

I think that would be best, yes. Could you share a simple block-diagram of the 
hardware ? It would help turning my random advices into semi-random advices 
:-)

-- 
Regards,

Laurent Pinchart