Re: [PATCH RFC 0/3] Add splash DRM client

[Francesco Valla <francesco@valla.it>]

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Hi Thomas,

On Monday, 27 October 2025 at 13:35:31 Thomas Zimmermann <tzimmermann@suse.de> wrote:
> Hi Francenso, Maxime,
> 
> Am 27.10.25 um 11:09 schrieb Maxime Ripard:
> > 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.
> 
> The panic screen has configurable foreground/background colors. Maybe we 
> can harmonize these settings.
> 

Maybe, but probably the panic colors would typically be much more vibrant
than splash ones. 

> >
> >>    - a single-line text message, which can be set through sysfs or
> >>      directly from the kernel command line;
> 
> Put it into the kernel config.
> 
> >>    - a very simple progress bar, which can be driven through sysfs;
> 
> Once you have options to control these settings from user space, you 
> should do it in user space entirely. As Maxime suggested, please improve 
> plymouth for anything with animation.
> 

On this I can agree, see my reply to Maxime.

> >>    - a static image (optional).
> 
> Board vendors often provide an image, see /sys/firmware/acpi/bgrt/. This 
> is a candidate for display, or the penguin or a custom image. Please 
> make it configurable by Kconfig. Again, if you need policy and 
> heuristics for deciding what to display, you better do this in user space.
>

I'm not under ACPI/UEFI typically, and the concept for this patch was not
developed on such system. But I'll take a look!

> > 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.
> 
> Agreed. With an updater running in user space, that process should also 
> manage the display update. No need for this in the kernel.
> 
> >
> >> 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.
> 
> file /sys/firmware/acpi/bgrt/image
> /sys/firmware/acpi/bgrt/image: PC bitmap, Windows 3.x format, 768 x 256 
> x 24, image size 589824, cbSize 589878, bits offset 54
> 
> That should probably be the format for now unless your firmware uses 
> something else natively. Code for reading a BMP file can be found in the 
> efifb driver. [1]
> 
> [1] 
> https://elixir.bootlin.com/linux/v6.17.5/source/drivers/video/fbdev/efifb.c#L24
> 

When I started working on the patch I was not able to find this BMP decoder,
I only found the PPM one from the bootup logo. I'll take a look here too.  


> Apart from the criticism for complexity, I do like the idea of having a 
> splash screen.
> 
> Best regards
> Thomas
> 
> >
> > Maxime
> 
> 

Thank you!

Best regards,
Francesco

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