linux-kernel - Re: [PATCH 2/5] gpu: drm: adp: Add Apple Display Pipe driver

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Message-ID: <cc71021e-b53d-4eda-bad8-abb4df13575f@marcan.st>
Date: Mon, 25 Nov 2024 23:24:25 +0900
From: Hector Martin <marcan@...can.st>
To: Sasha Finkelstein <fnkl.kernel@...il.com>, neil.armstrong@...aro.org
Cc: Sven Peter <sven@...npeter.dev>, Alyssa Rosenzweig
 <alyssa@...enzweig.io>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Jessica Zhang <quic_jesszhan@...cinc.com>,
 asahi@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
 dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, Janne Grunau <j@...nau.net>
Subject: Re: [PATCH 2/5] gpu: drm: adp: Add Apple Display Pipe driver

On 2024/11/25 20:24, Sasha Finkelstein wrote:
> On Mon, 25 Nov 2024 at 09:50, Neil Armstrong <neil.armstrong@...aro.org> wrote:
>>
>> So this controller only supports a single mode ???????
>>
> Most likely. On all devices it is connected to a single built-in display.

More specifically, the controller obviously supports multiple modes but
it is pre-initialized by the bootloader for the single hardwired
display's only mode. So as far as the driver is concerned, there is a
single possible mode, and there's no point in trying to be more generic
if there is no hardware that would use that.

In general, it is not possible/practical to be generic for reverse
engineered hardware with no specs documenting how to drive it
generically. You just can't know how to implement the options that are
never used in practice. I spent a lot of time on exceptions to this rule
for the GPIO and SPI controllers, and that's not going to happen for
more complex hardware like MIPI DSI.

- Hector