[Pkg-raspi-maintainers] Bug#1035878: rpi400: visual speckling on 'faulty' HDMI port during mouse movement

[James Addison]

Package: raspi-firmware
Severity: normal

Dear Maintainer,

I've installed a RPi 400 system using a regular build of Debian Installer
(bookworm RC2), and have begun using the official RPi firmware (as distributed
in the 'raspi-firmware' bookworm package - including bootcode.bin) to start the
the kernel.

During an X-based LXDE session, I see visual 'speckling' (static-like noise
that blits across the lower half-or-so of the display, mostly with dark
greenish and dark bluish pixels) when moving the mouse pointer.


NOTE: There are _two_ micro-HDMI ports on the RPi 400.  This problem only
occurs when the HDMI cable is connected to the port closest to the keyboard
indicator lights (capslock, numlock).  The problem _does not_ occur when the
HDMI cable is connected to the port closest to the USB-C power supply port.


Repro details:

Moving the pointer through particular areas of the display seem to cause the
flickering/speckling more repeatably.  The lower 40-or-so pixels of the screen
in particular (where the LXDE menubar appears by defaul) seems to be a boundary
that causes the problem fairly reliably.

Note that when pointer/cursor movement _stops_, the visual corruption also
stops occurring.  In other words: the condition appears on-screen temporarily
during the mouse pointer movement.


Additional details:

  * The system is running Debian kernel 6.1.0-8 from linux-image-arm64
    6.1.25-1

  * The problem is also replicable using the latest '*.{elf,dat,bin}' files
    from the upstream raspberrypi/firmware version 1.20230405 release.

  * The devicetree 'compatible' entry for the gpu is:

  $ cat /proc/device-tree/gpu/compatible
  brcm,bcm2711-vc5

  * The vc4 video driver appears to initialize fine according to dmesg (I don't
    see any errors).

  * The vc4 driver is in use as a framebuffer device, I think.

  $ dmesg --notime | grep -i vc4 | tail -n 2
  [drm] Initialized vc4 0.0.0 20140616 for gpu on minor 1
  vc4-drm gpu: [drm] fb0: vc4drmfb frame buffer device

  * I've attempted to use the 'config_hdmi_boost'[1] setting from the upstream
    firmware to resolve the problem, since it mentions speckling (in fact, it's
    where I learned the term 'speckling') specifically, but this has not had
    any effect.  The documentation there does mention that the option is
    ignored on RPi4, and it seems possible that that may also apply to RPi400.

  * Reducing the resolution of the desktop does not reduce or eliminate the
    speckling.


Thank you for any help / suggestions,
James

[1] - https://www.raspberrypi.com/documentation/computers/config_txt.html#config_hdmi_boost