Regression in drm: drm/fbdev doesn't initialize if set to triple buffer

[mohammad.rasim96@gmail.com (MOHAMMAD RASIM)]

Hi,

I've an issue where the drm_meson driver doesn't work, it used to work 
in earlier 4.18 release and stopped working after about 4.18-rc3.

My board is videostrong k2 pro (S905 SoC), I use the meson-gxbb-p201 
device tree.

on the latest 4.19 tree the board boots fine but the drm driver doesn't 
work, and spits the following error in dmesg

[??? 2.489825] meson-drm d0100000.vpu: Queued 3 outputs on vpu
[??? 2.494685] meson-drm d0100000.vpu: Failed to create d0100000.vpu 
debugfs directory
[??? 2.502051] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[??? 2.508668] [drm] No driver support for vblank timestamp query.
[??? 2.573756] meson-drm d0100000.vpu: bound c883a000.hdmi-tx (ops 
meson_dw_hdmi_ops)
[??? 2.695917] meson-drm d0100000.vpu: [drm:drm_fb_helper_fbdev_setup] 
*ERROR* Failed to set fbdev configuration
[??? 2.700310] meson-drm d0100000.vpu: master bind failed: -22
[??? 2.705747] meson-drm: probe of d0100000.vpu failed with error -22

Digging deeper into the problem revealed that the problem is related to 
the CONFIG_DRM_FBDEV_OVERALLOC option, If I set it to 100 or 200 it 
works fine and the drm driver works ( or by passing 
drm_kms_helper.drm_fbdev_overalloc=200 in the bootargs). The problem 
reappears when setting it to 300.

Bisecting shows that the first bad commit that introduced this 
regression is this: 244007ecb6bb94fa4e9b9a969fa86f2ad86ec543

Another problem that I face and might be related is that the sometimes 
the boot get stuck on the following lines:

[??? 2.501539] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[??? 2.508158] [drm] No driver support for vblank timestamp query.

This problem is random and can't always reproduced, sometimes it takes a 
few reboots to get the board to boot pass those lines.

Regards