drm/panic: Add a drm panic handler

This introduces a new drm panic handler, which displays a message when a panic occurs.
So when fbcon is disabled, you can still see a kernel panic.

This is one of the missing feature, when disabling VT/fbcon in the kernel:
https://www.reddit.com/r/linux/comments/10eccv9/config_vt...
Fbcon can be replaced by a userspace kms console, but the panic screen must be done in the kernel.

This is a proof of concept, and works with simpledrm and mgag200, using a new get_scanout_buffer() api

To test it, make sure you're using the simpledrm driver, and trigger a panic:
echo c > /proc/sysrq-trigger

v2:
 * Use get_scanout_buffer() instead of the drm client API. (Thomas Zimmermann)
 * Add the panic reason to the panic message (Nerdopolis)
 * Add an exclamation mark (Nerdopolis)
 
v3:
 * Rework the drawing functions, to write the pixels line by line and
 to use the drm conversion helper to support other formats.
 (Thomas Zimmermann)
 
v4:
 * Fully support all simpledrm formats using drm conversion helpers
 * Rename dpanic_* to drm_panic_*, and have more coherent function name.
   (Thomas Zimmermann)
 * Use drm_fb_r1_to_32bit for fonts (Thomas Zimmermann)
 * Remove the default y to DRM_PANIC config option (Thomas Zimmermann)
 * Add foreground/background color config option
 * Fix the bottom lines not painted if the framebuffer height
   is not a multiple of the font height.
 * Automatically register the driver to drm_panic, if the function
   get_scanout_buffer() exists. (Thomas Zimmermann)
 * Add mgag200 support.
 
With mgag200 support, I was able to test that the xrgb8888 to rgb565 conversion is working.

A few more though:
 1) what about gpu with multiple monitor connected ?
maybe get_scanout_buffer() could return a list of scanout buffers ?
 2) I think for some GPU drivers, there might need a flush_scanout_buffer() function, that should be called after the scanout buffer has been filled ?


Best regards,


Jocelyn Falempe (4):
  drm/format-helper: Export line conversion helper for drm_panic
  drm/panic: Add a drm panic handler
  drm/simpledrm: Add drm_panic support
  drm/mgag200: Add drm_panic support

 drivers/gpu/drm/Kconfig               |  22 ++
 drivers/gpu/drm/Makefile              |   1 +
 drivers/gpu/drm/drm_drv.c             |   8 +
 drivers/gpu/drm/drm_format_helper.c   |  88 +++++-
 drivers/gpu/drm/drm_panic.c           | 413 ++++++++++++++++++++++++++
 drivers/gpu/drm/mgag200/mgag200_drv.c |  24 ++
 drivers/gpu/drm/tiny/simpledrm.c      |  15 +
 include/drm/drm_drv.h                 |  14 +
 include/drm/drm_format_helper.h       |   9 +
 include/drm/drm_panic.h               |  41 +++
 10 files changed, 627 insertions(+), 8 deletions(-)
 create mode 100644 drivers/gpu/drm/drm_panic.c
 create mode 100644 include/drm/drm_panic.h


base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
-- 
2.41.0