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

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Message-ID: <87pof0y5ra.fsf@eliezer.anholt.net>
Date:   Mon, 22 May 2017 13:51:37 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Archit Taneja <architt@...eaurora.org>
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>,
        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.

Laurent Pinchart <laurent.pinchart@...asonboard.com> writes:

> Hi Archit,
>
> On Friday 19 May 2017 14:24:36 Archit Taneja wrote:
>> On 05/18/2017 08:25 PM, Laurent Pinchart wrote:
>> > On Thursday 18 May 2017 13:56:19 Archit Taneja wrote:
>> >> On 05/17/2017 12:16 AM, Eric Anholt wrote:
>> >
>> > [snip]
>> > 
>> >>> In terms of physical connections:
>> >>>    [15-pin "DSI" connector on 2835]
>> >>>     | I2C               | DSI
>> >>>    / \        SPI       |
>> >>> [TS]  [Atmel]------[TC358762]
>> >>>        \                |
>> >>>         \PWM            |
>> >>>          \              | DPI
>> >>> 
>> >>> [some backlight]------[some unknown panel]
>> >>> 
>> >>> The binding I'm trying to create is to expose what's necessary for a
>> >>> driver that talks I2C to the Atmel, which then controls the PWM and does
>> >>> the command sequence over SPI to the Toshiba that sets up its end of the
>> >>> DSI link.
>> >> 
>> >> The bridge (Atmel + TC358762 combination) here looks like it's primarily
>> >> an i2c device (i.e, the control bus is i2c). Therefore, the drm-bridge
>> >> driver here should be an i2c driver instead of a mipi_dsi_driver.
>> > 
>> > Glad to see we agree, that's what I've proposed in a separate answer :-)
>> > I'd go one step further though, there should be no DRM bridge, just a DRM
>> > panel.
>>
>> If the PCB containing the controller chips and the panel are part of a
>> single casing, and the set up won't work with another panel, then yeah, I
>> agree. If the bridge chips are on a separate adapter board, and there is a
>> possibility to connect other panels, then maybe a separate DRM bridge and a
>> DRM panel might be a safer bet.
>
> I thought it was a single black box, but upon closer inspection there's a 
> separate PCB with the Microcontroller and TC358762.
>
> Eric, do you know if it's possible to exchange the panel for another one (and 
> not just an model with identical features from another vendor, but another 
> panel with a different mode for instance) without reprogramming the 
> microcontroller, or is the bridge board tied to the panel model ?

Not without finding some other panel with equivalent non-standard
connectors / doing your own soldering, at a minimum.  And we don't know
what kind of programming the microcontroller does, since it's a black
box.

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