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From:   Alistair Francis <alistair23@gmail.com>
Date:   Sun, 15 Aug 2021 22:16:47 +1000
Message-ID: <CAKmqyKMLyx+CvBQbLz-xQvwLS692tx-4xOgU7b-V2J676D29yg@mail.gmail.com>
Subject: Revert "video: fbdev: mxsfb: Remove driver"
To:     Fabio Estevam <festevam@gmail.com>,
        Shawn Guo <shawnguo@kernel.org>,
        Sascha Hauer <kernel@pengutronix.de>,
        dl-linux-imx <linux-imx@nxp.com>, Marek Vasut <marex@denx.de>,
        b.zolnierkie@samsung.com,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alistair Francis <alistair@alistair23.me>
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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

Alistair