Message-ID: <yq4btdc5qqukuqps7y53dratmu64ghyifgprlndnk5rbgml4of@rvca75sncvsm>
Date: Mon, 27 Oct 2025 11:09:56 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Francesco Valla <francesco@...la.it>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Jonathan Corbet <corbet@....net>, 
	Jocelyn Falempe <jfalempe@...hat.com>, Javier Martinez Canillas <javierm@...hat.com>, 
	Sam Ravnborg <sam@...nborg.org>, linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linux-doc@...r.kernel.org, linux-embedded@...r.kernel.org
Subject: Re: [PATCH RFC 0/3] Add splash DRM client

Hi,

On Mon, Oct 27, 2025 at 12:03:00AM +0100, Francesco Valla wrote:
> this patchset adds a new DRM client offering splash functionalities,
> able to draw to screen:
> 
>   - a colored background;

So, I like that part, and we were recently discussing about this.

>   - a single-line text message, which can be set through sysfs or
>     directly from the kernel command line;
>   - a very simple progress bar, which can be driven through sysfs;
>   - a static image (optional).

But there's no reason to have all that in the kernel, and we already
have userspace components to do so (plymouth being the main "mainstream"
one).

> Once compiled inside the kernel, the client can be enabled through the
> command line specifying the drm_client_lib.active=splash parameter.
> 
> == Motivation ==
> 
> The motivation behind this work is to offer to embedded system
> developers a new path for a simple activation of the display(s)
> connected to their system, with the following usecases:
> 
>   - bootsplash - possibly displaying even before init;
>   - early activation of the display pipeline, in particular whenever one
>     component of the pipeline (e.g.: a panel) takes a non-negligible
>     time to initialize;
>   - recovery systems, where the splash client can offer a simple feedback
>     for unattended recovery tasks;
>   - update systems, where the splash client can offer a simple feedback
>     for unattended update tasks.

If plymouth cannot be used by embedded systems for some reason, then you
should work on a plymouth alternative.

> While the first seems the most obvious one, it was the second that acted
> as the driver, as in the past I had to implement a ugly workaround using
> a systemd generator to kickstart the initialization of a display and
> shave ~400ms of boot time.
> 
> The last 2 usecase, instead, are the reason I dropped the "boot" part
> from bootsplash.
> 
> == Implementation details ==
> 
> The design is quite simple, with a kernel thread doing the heavylifting
> for the rendering part and some locking to protect interactions with it.
> 
> The splash image is loaded using the firmware framework, with the client
> expecting to find a binary dump having the right dimensions (width and
> height) and FOURCC format for each modeset. Given a 1920x1080 RGB888
> modeset, the client will for example search for a firmware named:
> 
>    drm_splash_1920x1080_RG24.raw
> 
> If the firmware cannot be loaded directly, the NOUEVENT sysfs fallback
> mechanism is used to let userspace load the appropriate image.
> 
> == Testing ==
> 
> Testing was done on qemu (both with vkms and bochs drivers), on a HDMI
> display connected to a Beagleplay and on a ILI9341 SPI display connected
> to a i.MX93 FRDM board. All these platforms revealed different
> weaknesses that were hopefully removed.
> 
> == Open points / issues ==
> 
> The reason for this being an RFC is that there are several open points:
> 
>   - Support for tiled connectors should be there, but has not been
>     tested. Any idea on how to test it?

Did you mean tiled formats?

>   - I'm not entirely convinced that using the firmware framework to load
>     the images is the right path. The idea behind it was to re-use the
>     compressed firmware support, but then I discovered it is not there
>     for built-in firmware.

Yeah, firmware loading for this has a few issues (being tedious to setup
for when built-in being one). I think just going the fbdev penguin road
is a better choice: you provide the path, and it's embedded in the
kernel directly.

>   - Again on the firmware loading: CONFIG_LOADPIN would interfere with
>     sysfs loading.
>   - And again: FW_ACTION_NOUEVENT only has one user inside the kernel,
>     leading me to think it is de-facto deprecated. And still, uevents
>     for firmware loading seem frowned upon these days... 
>   - Generating binary dumps for... basically any format is not so
>     straightforward. I crafted a Python tool with AI help which seems
>     to work quite well, but I honestly did not yet understood which is
>     the policy for AI-generated code inside the kernel, so it is not
>     included in this patch set. All client code is genuine, though.

BMP is simple enough to support so we should probably use that instead
of a custom format.

Maxime

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