Hi,
I've been working on a tinydrm-based driver for black/white/red, b/w/yellow and b/w epaper displays made by good display[sic!][0]. These panels are fairly popular since waveshare[1] sells raspberry pi and arduino-compatible breakout boards with them.
The current state of the art in open source software as well as the vendors examples on how to drive these seems to be "pour some fairy dust into /dev/mem". This cannot stand, so here is a (tested, working) tinydrm driver. I've got working color output on two black/white/red panels (2.7" and 4.2") from python[2], and I have a kernel console showing up[3,4] and I can print colorful cows using cowsay and some ANSI escape sequences[5].
In this rfc patch I've added a couple ioctls to allow userspace to control refresh behavior. IMO this is necessary for a fully working driver: These displays support partial refresh, and you can use that to do proper animations[6]. But for this to work, you need to fine-tune the display's driving waveforms depending on the content displayed.
I have a couple of questions regarding this driver, thus the RFC: (1) By default, when loading the module a vt console binds to the fb. I think this is useful, but the cursor blink of the vt leads to an eternal refresh loop. Refresh on these displays is *extremely* slow (1 frame per 15s), so ideally I'd want the cursor to not blink. Is there some nice way to tell the vt/console driver to do this by default? (2) Is ioctl the correct interface for the driving waveform/refresh stuff? (3) I read that any drm driver has to be committed along with a libdrm implementation. I think the most likely interface for anyone to interact with this driver would be the fb dev. Do I have to make some userspace implementation as well anyway? If so, where would that go: libdrm or some sort of new libepaper? (4) The driver accepts both XRGB8888 and RGB565 (for compatibility with small LCDs). The driver's current approach to calculate a b/w/r "ternary" image from this data is to just take the MSBs of each color component, then make anything red (R>127, G,B<=127) red, anything black (R,G,B each <=127) black and anything else white. This is since the display's default state is white, and a pixel can turn either red or black from that. Note that it's the actual pixel changing color, i.e. there is no black and red sub-pixels. If you try to drive black and red at the same time, the chip just selects red for that pixel. What are your thoughts on this interface? I was thinking about using some indexed color mode, but that would limit possible future grayscale support[7].
The following patches all apply on v5.2. This is my first linux driver, so please be gentle but please do point out all my mistakes :) I'm aware the dt binding doc is still lacking.
Best regards, Jan Sebastian Götte
[0] https://www.good-display.com/ [1] https://www.waveshare.com/product/modules/oleds-lcds/e-paper.htm [2] https://blog.jaseg.net/images/epaper3.jpg [3] https://blog.jaseg.net/images/epaper1.jpg [4] https://blog.jaseg.net/images/epaper2.jpg [5] https://blog.jaseg.net/images/epaper4.jpg [6] https://www.youtube.com/watch?v=MsbiO8EAsGw [7] https://github.com/zkarcher/FancyEPD