Questions about KMS flip

[Michel Dänzer]

On 2021-11-12 16:03, Christian König wrote:
> Am 12.11.21 um 15:30 schrieb Michel Dänzer:
>> On 2021-11-12 15:29, Michel Dänzer wrote:
>>> On 2021-11-12 13:47, Christian König wrote:
>>>> Anyway this unfortunately turned out to be work for Harray and Nicholas. In detail it's about this bug report here: https://bugzilla.kernel.org/show_bug.cgi?id=214621
>>>>
>>>> Lang was able to reproduce the issue and narrow it down to the pin in amdgpu_display_crtc_page_flip_target().
>>>>
>>>> In other words we somehow have an unbalanced pinning of the scanout buffer in DC.
>>> DC doesn't use amdgpu_display_crtc_page_flip_target AFAICT. The corresponding pin with DC would be in dm_plane_helper_prepare_fb, paired with the unpin in
>>> dm_plane_helper_cleanup_fb.
>>>
>>>
>>> With non-DC, the pin in amdgpu_display_crtc_page_flip_target is paired with the unpin in dm_plane_helper_cleanup_fb
>> This should say amdgpu_display_unpin_work_func.
> 
> Ah! So that is the classic (e.g. non atomic) path?

Presumably.


>>> & dce_v*_crtc_disable. One thing I notice is that the pin is guarded by if (!adev->enable_virtual_display), but the unpins seem unconditional. So could this be about virtual display, and the problem is actually trying to unpin a BO that was never pinned?
> 
> Nope, my educated guess is rather that we free up the BO before amdgpu_display_unpin_work_func is called.
> 
> E.g. not pin unbalance, but rather use after free.

amdgpu_display_crtc_page_flip_target calls amdgpu_bo_ref(work->old_abo), and amdgpu_display_unpin_work_func calls amdgpu_bo_unref(&work->old_abo) only after amdgpu_bo_unpin. So what you describe could only happen if there's an imbalance elsewhere such that amdgpu_bo_unref is called more often than amdgpu_bo_ref, or maybe if amdgpu_bo_reserve fails in amdgpu_display_unpin_work_func (in which case the "failed to reserve buffer after flip" error message should appear in dmesg).


-- 
Earthling Michel Dänzer            |                  https://redhat.com
Libre software enthusiast          |         Mesa and Xwayland developer