linux-kernel - Re: Future handling of complex RGB devices on Linux v2

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Message-ID: <ZdeGiMf2npmzJidU@duo.ucw.cz>
Date: Thu, 22 Feb 2024 18:38:16 +0100
From: Pavel Machek <pavel@....cz>
To: Pekka Paalanen <pekka.paalanen@...oniitty.fi>
Cc: Werner Sembach <wse@...edocomputers.com>,
	Hans de Goede <hdegoede@...hat.com>, Lee Jones <lee@...nel.org>,
	jikos@...nel.org, linux-kernel@...r.kernel.org,
	Jelle van der Waa <jelle@...aa.nl>,
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	linux-input@...r.kernel.org, ojeda@...nel.org,
	linux-leds@...r.kernel.org
Subject: Re: Future handling of complex RGB devices on Linux v2

Hi!

> > > so after more feedback from the OpenRGB maintainers I came up with an even
> > > more generic proposal:
> > > https://gitlab.com/CalcProgrammer1/OpenRGB/-/issues/3916#note_1753072869  
> > 
> > > >evaluate-set-command ioctl taking:
> > > >{
> > > >    enum command                /* one of supported_commands */
> > > >    union data
> > > >    {
> > > >        char raw[3072],
> > > >        {
> > > >            <input struct for command 0>
> > > >        },  
> > 
> > Yeah, so ... this is not a interface. This is a backdoor to pass
> > arbitrary data. That's not going to fly.
> > 
> > For keyboards, we don't need complete new interface; we reasonable
> > extensions over existing display APIs -- keyboards are clearly 2D.
> 
> I suppose they could be seen as *a* display, but if you are referring
> to DRM KMS UAPI, then no, I don't see that fitting at all:

So -- we already have very similar displays in
drivers/auxdisplay. drivers/auxdisplay/cfag12864b.c is 128x64 display,
1-bit display for example.

> - the "pixel grid" is not orthogonal, it's not a rectangle, and it
>   might not be a grid at all

It is quite close to orthogonal. I'd suggest simply pretending it is
orthogonal grid with some pixels missing :-). We already have
cellphone displays with rounded corners and holes in them, so I
suspect handling of missing pixels will be neccessary anyway.

> - Timings and video modes? DRM KMS has always been somewhat awkward for
>   display devices that do not have an inherent scanout cycle and timings
>   totally depend on the amount of pixels updated at a time
>   (FB_DAMAGE_CLIPS), e.g. USB displays (not USB-C DP alt mode).
>   They do work, but they are very different from the usual hardware
>   involved with KMS, require special consideration in userspace, and
>   they still are actual displays while what we're talking about here
>   are not.

As you say, there are other displays with similar problems.

> - KMS has no concept of programmed autonomous animations, and likely
>   never will. They are not useful with actual displays.

Yep. We need some kind of extension here, and this is likely be quite
vendor-specific, as animations will differ between vendors. I guess
"please play pattern xyzzy with parametrs 3 and 5" might be enough for this.

> - Userspace will try to light up KMS outputs automatically and extend
>   the traditional desktop there. This was already a problem for
>   head-mounted displays (HMD) where it made no sense. That was worked
>   around with an in-kernel list of HMDs and some KMS property
>   quirking.

This will need fixing for cfag12864b.c, no? Perhaps userspace should
simply ignore anything smaller than 240x160 or something...

> Modern KMS UAPI very much aims to be a generic UAPI that abstracts
> display devices. It already breaks down a little for things like USB
> displays and virtual machines (e.g. qemu, vmware, especially with
> remote viewers), which I find unfortunate. With HMDs the genericity
> breaks down in other ways, but I'd claim HMDs are a better fit still
> than full-featured VM virtual displays (cursor plane hijacking). With
> non-displays like keyboards the genericity would be completely lost, as
> they won't work at all the same way as displays. You cannot even show
> proper images there, only coarse light patterns *IF* you actually know
> the pixel layout. But the pixel layout is(?) hardware-specific which is
> the opposite of generic.
> 
> While you could dress keyboard lights etc. up with DRM KMS UAPI, the
> userspace would have to be written from scratch for them, and you
> somehow need to make existing KMS userspace to never touch those
> devices. What's the point of using DRM KMS UAPI in the first place,
> then?

Well, at least we have good UAPI to work with. Other options were 100
files in /sys/class/leds, pretending it is a linear array of leds,
just passing raw data around, and pretending it is grid of RGB888
data.

Anyway, there are devices such as these: (8x8 LED display).

https://www.laskakit.cz/8x8-led-matice-s-max7219-3mm-cervena/

We should think about what interface we want for these, and then I
believe we should make RGB keyboards use something similar.

Best regards,
								Pavel
-- 
People of Russia, stop Putin before his war on Ukraine escalates.

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