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

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Message-ID: <87h90ku4sq.fsf@eliezer.anholt.net>
Date:   Tue, 16 May 2017 09:47:49 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        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\@vger.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] dt-bindings: Document the Raspberry Pi Touchscreen nodes.

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-touchscr
>>>> een-bridge.txt create mode 100644
>>>> Documentation/devicetree/bindings/display/panel/raspberrypi,7inch-touchscre
>>>> en-panel.txt
>>>>
>>>> diff --git
>>>> a/Documentation/devicetree/bindings/display/bridge/raspberrypi,7inch-touchs
>>>> creen-bridge.txt
>>>> b/Documentation/devicetree/bindings/display/bridge/raspberrypi,7inch-touchs
>>>> creen-bridge.txt new file mode 100644
>>>> index 000000000000..a5669beaf68f
>>>> --- /dev/null
>>>> +++
>>>> b/Documentation/devicetree/bindings/display/bridge/raspberrypi,7inch-touchs
>>>> creen-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?

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