[PATCH v2 0/2] Fix SDL 1.2 on emulated fbdev devices (broken in kernels >=4.19)

[Ivan Mironov <mironov.ivan@gmail.com>]

Hi,

Originally this issue was brought up on linux.org.ru forum by user
saahriktu, he is on Cc. He discovered that commit db05c48197759
("drm: fb-helper: Reject all pixel format changing requests") breaks
support of SDL1 programs, like various old games and emulators of old
game consoles. First patch contains fix for that commit.

I tried to reproduce the same issue in a VM under qemu, and found yet
another part of kernel code which prevents SDL1 apps from running
normally. Second patch in this series fixes this problem.

Also, it seems that at least in some cases both problems could be
circumvented by adding appropriate modes into /etc/fb.modes. But without
examining the kernel code it is not clear which values are correct. I am
not sure that such circumvention covers all possible cases, and it is
definitely far from any user-friendliness.

First patch in this series fixes a clear regression. Second patch is
optional, please read commit message carefully before applying it.

Changes in v2:
- Added "Cc: stable" to the second patch.
- Proposed by Daniel Vetter: always use current depth (fb->format->depth)
  in a case of zero pixel format values and do not perform any guessing.

Changes in v1:
- Added "Cc: stable" to the patch which fixes known regression.
- Added more information and detailed reproduction steps in commit
  messages.

Changes in v0:
- RFC patch series introduced.

Ivan Mironov (2):
  drm/fb-helper: Partially bring back workaround for bugs of SDL 1.2
  drm/fb-helper: Ignore the value of fb_var_screeninfo.pixclock

 drivers/gpu/drm/drm_fb_helper.c | 149 ++++++++++++++++++++------------
 1 file changed, 95 insertions(+), 54 deletions(-)

-- 
2.20.1