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

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Message-ID: <48f3bb08-a591-401c-bc0c-6af0506a1b1f@linaro.org>
Date: Mon, 25 Nov 2024 16:37:35 +0100
From: neil.armstrong@...aro.org
To: Hector Martin <marcan@...can.st>,
 Sasha Finkelstein <fnkl.kernel@...il.com>
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

Hi Hector,

On 25/11/2024 15:24, Hector Martin wrote:
> 
> 
> 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.

I totally got the point, I just asked to slightly change the design to fit
the overall DRM/DSI/Panel architecture.

> 
> 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.

I'm fine with the overall actual design since it's a special usecase
for a special display present on only a few SKUs.

Nevertheless, as Maxime expressed, you should still try to fit nicely
on the abstractions, or change the abstractions.

Neil

> 
> - Hector
>