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

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Date:   Tue, 17 Aug 2021 12:03:46 +0200
From:   Marek Vasut <marex@...x.de>
To:     Alistair Francis <alistair23@...il.com>
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 8/17/21 11:08 AM, Alistair Francis wrote:
> On Mon, Aug 16, 2021 at 5:55 PM Marek Vasut <marex@...x.de> wrote:
>>
>> On 8/16/21 9:34 AM, Alistair Francis wrote:
>>> 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.
>>
>> 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.