Re: Future handling of complex RGB devices on Linux v2

Thomas Zimmermann
2024-02-23T09:21:03Z

Hi

Am 22.02.24 um 18:38 schrieb Pavel Machek:
> 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],
>>>>>         {
>>>>>
>>>>>         },
>>> 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.

Auxdisplay can be anything for everyone. It appears to be the rest that
didn't clearly fit elsewhere. The core interface seems to be a custom
char device. The fbdev code in cfag12864b is not representative.

>
>> - 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.

The litmus test for DRM and fbdev is something like "would the user run
the console, desktop, or any other meaningful output in this display".
That is also what userspace (e.g., X, Wayland, gfx terminals) expects: a
display to show the user's main output. Keyboard LEDs don't fit here.

Best regards
Thomas

>
>> - 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

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)