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Introduction

This module provides userspace damage tracking for (at least some) i.MX6 e-paper framebuffers. It has been tested on the reMarkable tablet which uses an i.MX6 SoloLite.

Since e-paper displays have very high updates latencies, which depend on the size of the region updated, well-written applications intended to run on such displays will, whenever ready to draw to the display, inform the framebuffer driver of precisely what areas of the display need to be updated. Consequently, the kernel has detailed damage tracking information available.

For some applications, damage-tracking information is useful, and the lack of efficient damage-tracking can be an issue when using the Linux framebuffer. Since, for e-paper displays, the kernel actually has the necessary information available, this module exports that information to userspace for the use of programs. The reference consumer of the information from this module is rM-vnc-server, an efficient VNC server for the reMarkable tablet.

Interface

When the module is loaded, it will inject damage recording infrastructure to a framebuffer device /dev/fbn specified by the fbnode module parameter (which defaults to 0). It will then create a /dev/fbdamage device, on which reads will block until damage is created. Each call to read will return a single struct mxcfb_damage_update. The overflow_notify member will be set if any updates have been discarded, and the data member will be set to the original mxcfb_update_data passed to the kernel in an MXCFB_SEND_UPDATE ioctl.

Building

The supported way to build this is via the Nix package manager, through the nix-remarkable expressions. To build just this project via nix build from this repo, download it into the pkgs/ directory of nix-remarkable.

For any other system, this should build like any other Linux kernel module being cross-built for the reMarkable.

Prebuilt binaries are available in the Releases tab.

Usage

Copy mxc_epdc_fb_damage.ko to the reMarkable and run insmod on it to load. If your platform has an mxc framebuffer numbered other than zero, pass fbnode=n to insmod.