linux-kernel - Re: [PATCH v2 1/2] dt-bindings: Add binding for Sitronix ST7735R display panels

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Date:   Mon, 11 Dec 2017 11:11:39 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     David Lechner <david@...hnology.com>
Cc:     "open list:DRM PANEL DRIVERS" <dri-devel@...ts.freedesktop.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Noralf Trønnes <noralf@...nnes.org>,
        limor@...yada.net, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] dt-bindings: Add binding for Sitronix ST7735R
 display panels

On Sun, Dec 10, 2017 at 11:10 PM, David Lechner <david@...hnology.com> wrote:

> This adds a new device tree binding for Sitronix ST7735R display panels,
> such as the Adafruit 1.8" TFT.
>
> Signed-off-by: David Lechner <david@...hnology.com>
> Acked-by: Rob Herring <robh@...nel.org>

(...)
> Limor brought up an interesting point in an off-list discussion. The
> same controller can be wired to the same LCD in different ways. This is
> evident in the fbftf drivers in staging. There is an "adafruit18" and an
> "adafruit18_green" in fbftf_devices.c where apparently, two otherwise
> identical displays were wired slightly differently at the factory so that
> on one, the on-board GRAM word 0 does not correspond to pixel 0,0 on the
> LCD. It requires a special offset to the GRAM starting address in order to
> have the image displayed correctly.
>
> Additionally, fbtft supports a SainSmart 1.8" TFT [3] that uses the same
> controller, but it appears that these have different gamma curves (perhaps
> they use different LCDs?). The available pins are exactly the same as the
> Adafruit display though.

This discussion came up in relation to the Ilitek ILI9322 bindings, here
are some links:
https://marc.info/?l=dri-devel&m=150300267811979&w=2
https://marc.info/?l=devicetree&m=150438702909506&w=2
https://marc.info/?l=dri-devel&m=150628542603682&w=2

The last mail from Rob clearly indicates that he doesn't like this kind
of open-ended configuration data stashed up in the DTS.

> There was a binding acked recently for the LCD on a D-Link DIR-685 Wireless
> N Storage Router [8]. This uses the compound compatible string of "dlink,
> dir-685-panel", "ilitek,ili9322". If we want to try to keep things
> consistent, perhaps I should be adopting this pattern as well?

I think so.

> And perhaps
> it would be better to use the better known vendor name instead of the
> obscure vendors from the datasheets that I have found? For example,
> "adafruit,618" "sitronix,st7735r" instead of "jianda,jd-t18003-t01"?

I don't know this, just that it should be:

"vendor,specific-system-config", "vendor,ip-part";

Yours,
Linus Walleij