[Bug 1914535] Re: PL110 8-bit mode is not emulated correctly

[Peter Maydell]

Anyway, on first investigation I'm not sure what QEMU's pl11x model is doing 
wrong. The 8-bit mode is a palette-based setup, where the guest must program in 
the RGB values it wants to use into the palatte registers as RGB555 data:
 
https://developer.arm.com/documentation/ddi0293/c/programmer-s-model/register-descriptions/256x16-bit-color-palette-registers?lang=en
and if you add some debug printing to QEMU you can see the guest writing in 
a variety of definitely-not-shades-of-grey values to the palette, so it's not 
surprising that it comes back as not-grey.

I think
https://elixir.bootlin.com/linux/latest/source/drivers/video/fbdev/amba-clcd.c#L343
is where the driver is writing to the palette and it definitely thinks the 
display is pseudocolor, not greyscale.

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https://bugs.launchpad.net/bugs/1914535

Title:
  PL110 8-bit mode is not emulated correctly

Status in QEMU:
  New

Bug description:
  When the emulated pl110/pl111 is switched programmatically to 8-bit
  color depth mode, the display is drawn green and blue, but the real
  PL110 displays grayscale in 8-bit mode.

  The bug appears in qemu-system-arm version 3.1.0 (Debian
  1:3.1+dfsg-8+deb10u8) and qemu-system-arm version 5.2.50
  (v5.2.0-1579-g99ae0cd90d).

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