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

[Marek Vasut]

On 8/17/21 11:08 AM, Alistair Francis wrote:
> On Mon, Aug 16, 2021 at 5:55 PM Marek Vasut <marex@xxxxxxx> wrote:
> > 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 ?
>
> I do and /dev/fb0 exists
>
> > What sort of messages do you get and from where ?
>
> This is the error I get:
>
> xochitl[252]: Error writing variable information: Invalid argument...

Some ioctl returns -EINVAL maybe ? strace would tell you.

> xochitl is the proprietary userspace code. I don't really have a good
> idea of what that error would mean.
>
> I also see this:
>
> Framebuffer has wrong id: "mxcfb"

Are you sure you're not confusing mxcfb (The freescale imx scanout 
engine) with mxsfb (The originally sgtl block, bought by freescale) ?

> > 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.
>
> I had a quick look at this.
>
> xochitl does a lot more than just controls the display, it interacts
> with lots of other hardware and strace produces a lot of logs. I also
> don't see the error when manually starting it, only at boot (but it
> still doesn't work).
>
> A quick run with
>
> strace -f xochitcl
>
> and I don't even see an access to /dev/fb0, so I'm not sure where the
> accesses are coming from.

You can try writing the output of strace to file (strace -o) for easier 
analysis. Then grep for either access to /dev/fb0 (or any symlink to it 
which might exist), or search for the mxcfb string, maybe the 
application aborts even before opening the fbdev due to some other problem.