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

From: Peter Maydell
Subject: [Bug 1914535] Re: PL110 8-bit mode is not emulated correctly
Date: Fri, 19 Feb 2021 17:18:32 -0000

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:
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
is where the driver is writing to the palette and it definitely thinks the 
display is pseudocolor, not greyscale.

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  PL110 8-bit mode is not emulated correctly

Status in QEMU:

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

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