Re: Revert "video: fbdev: mxsfb: Remove driver"

From: Marek Vasut
Date: Mon Aug 16 2021 - 03:56:13 EST

On 8/16/21 9:34 AM, Alistair Francis wrote:
On Sun, Aug 15, 2021 at 10:31 PM Marek Vasut <marex@xxxxxxx> wrote:

On 8/15/21 2:16 PM, Alistair Francis wrote:
Hello,

Commit f225f1393f034 "video: fbdev: mxsfb: Remove driver" removed the
mxsfb fbdev driver.

I am now working on getting mainline Linux running on the reMarkable 2
eInk reader [1]. Unfortunately the rM2 doesn't use the standard EPD
controller on the i.MX SoC, but instead uses the LCD controller.

eInk panels are complex to control and require writing temperature
dependent waveforms. As these waveforms are proprietary [2] the rM
team runs closed source software called SWTCON in userspace to control
the panel [3].

This closed source software expects the fbdev mxsfb driver and it
doesn't work with the DRM mxsfb driver (at least not that I can get it
to).

The NXP fork of Linux also re-adds the fbdev driver [4], so they also
see some uses for it.

I was wondering if the community would be open to re-adding the fbdev
mxsfb driver to mainline? It could be re-added with a different
dt-binding so that it is not used by default and only enabled for
boards when required (like for the rM2).

1: https://remarkable.com/store/remarkable-2
2: https://goodereader.com/blog/e-paper/e-ink-waveforms-are-a-closely-guarded-secret
3: https://remarkablewiki.com/tech/rm2_framebuffer
4: https://source.codeaurora.org/external/imx/linux-imx/log/drivers/video/fbdev/mxsfb.c?h=lf-5.10.35-2.0.0

+CC Sam.

What sort of special thing does your proprietary userspace do that
cannot be added to the DRM driver or the fbdev emulation (if needed) ?

It's hard to tell. When using the DRM driver I get cryptic errors
about the frame buffer not being available.

Do you have fbdev emulation enabled ? Does /dev/fbX exist ?

What sort of messages do you get and from where ?

You could run strace on the application to see how it communicates with the old driver via the ioctl interface and compare it with the fbdev emulation on the new driver, maybe there is some odd ioctl which is not emulated.

There is also NXP 5.10.35 fork, so you could try picking the fbdev driver from there and add printks/trace_printks/etc. into both that and the fbdev emulation code, to see whether how either is called and what is failing/missing in the emulation.

It's difficult to know
what is going on as I don't have access to any of the source. I
suspect the userspace code could be updated to use the DRM driver, but
we would need the reMarkable devs to do that.

There is some effort to re-implement the proprietary user space swtcon
(https://github.com/timower/rM2-stuff#swtcon), but it seems to have
stalled. It wouldn't be impossible to get swtcon to work with the DRM
driver, but it would require a very large amount of reverse
engineering, that probably will never happen.

I wanted to see what the thoughts were on re-adding the fbdev mxsfb
driver. The commit message just says that because there is a DRM
driver we no longer need the fbdev one, but here is a case for the
fbdev driver. I was thinking that continuing to support the fbdev
mxsfb driver wouldn't be too much of a maintenance burden (but that's
obviously up to you). The NXP tree also seems to think the fbdev
driver is worth keeping around.

I don't think the NXP tree is a particularly good example of best practice, they don't use the mxsfb in the first place, they wrote their own DRM driver for the LCDIF IP, and they also keep the fbdev driver around, so yes, they have three drivers for the same IP in different state of decay and with different problems, instead of one driver that has all the functionality and fixes. Sigh ...

I cannot decide on the fbdev thing, that's I think up to Sam. However, my suggestion would be to find out what is missing in the fbdev emulation and possibly fill that in, so we will have only one driver to support all the functionality.