It appears drm-next TTM cleanup broke something . . .

[Kevin Brace]

Hi Dave,

It is a little urgent, so I am writing this right now.
As usual, I pulled in DRM repository code for an out of tree OpenChrome DRM repository a few days ago.
While going through the changes I need to make to OpenChrome DRM to compile with the latest Linux kernel, I noticed that ttm_bo_init_mm() was discontinued, and it was replaced with ttm_range_man_init().
ttm_range_man_init() has a parameter called "bool use_tt", but honestly, I do not think it is functioning correctly.
If I keep "ttm_tt_create" member of ttm_bo_driver struct null by not specifying it, TTM still tries to call it, and crashes due to a null pointer access.
The workaround I found so far is to specify the "ttm_tt_create" member by copying bo_driver_ttm_tt_create() from drm/drm_gem_vram_helper.c.
This is what the call trace looks like without specifying the "ttm_tt_create" member (i.e., this member is null).

_______________________________________________
. . .
kernel: [   34.310674] [drm:openchrome_bo_create [openchrome]] Entered openchrome_bo_create.
kernel: [   34.310697] [drm:openchrome_ttm_domain_to_placement [openchrome]] Entered openchrome_ttm_domain_to_placement.
kernel: [   34.310706] [drm:openchrome_ttm_domain_to_placement [openchrome]] Exiting openchrome_ttm_domain_to_placement.
kernel: [   34.310737] BUG: kernel NULL pointer dereference, address: 0000000000000000
kernel: [   34.310742] #PF: supervisor instruction fetch in kernel mode
kernel: [   34.310745] #PF: error_code(0x0010) - not-present page
. . .
kernel: [   34.310807] Call Trace:
kernel: [   34.310827]  ttm_tt_create+0x5f/0xa0 [ttm]
kernel: [   34.310839]  ttm_bo_validate+0xb8/0x140 [ttm]
kernel: [   34.310886]  ? drm_vma_offset_add+0x56/0x70 [drm]
kernel: [   34.310897]  ? openchrome_gem_create_ioctl+0x150/0x150 [openchrome]
. . .
_______________________________________________

The erroneous call to  "ttm_tt_create" member happens right after TTM placement is performed (openchrome_ttm_domain_to_placement()).
Currently, OpenChrome DRM's TTM implementation does not use "ttm_tt_create" member, and this arrangement worked fine until Linux 5.9's drm-next code.
It appears that Linux 5.10's drm-next code broke the code.

Regards,

Kevin Brace
Brace Computer Laboratory blog
https://bracecomputerlab.com