linux-kernel - Re: Revert "video: fbdev: mxsfb: Remove driver"

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Date:   Mon, 16 Aug 2021 17:34:17 +1000
From:   Alistair Francis <alistair23@...il.com>
To:     Marek Vasut <marex@...x.de>
Cc:     Fabio Estevam <festevam@...il.com>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <kernel@...gutronix.de>,
        dl-linux-imx <linux-imx@....com>, b.zolnierkie@...sung.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alistair Francis <alistair@...stair23.me>,
        Sam Ravnborg <sam@...nborg.org>
Subject: Re: Revert "video: fbdev: mxsfb: Remove driver"

On Sun, Aug 15, 2021 at 10:31 PM Marek Vasut <marex@...x.de> 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. 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.

Alistair