Questions about KMS flip

[Michel Dänzer]

On 2021-11-15 07:41, Lang Yu wrote:
> On Fri, Nov 12, 2021 at 05:10:27PM +0100, Michel DDDnzer wrote:
>> 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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.kernel.org%2Fshow_bug.cgi%3Fid%3D214621&data=04%7C01%7CLang.Yu%40amd.com%7Cca557eab16864ab544a108d9a5f6f288%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637723302338811983%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=9oG6k22BVp%2BkSvRX6uMlGXc6G%2BA5UL0Qy3W5iDTpvzs%3D&reserved=0
>>>>>>
>>>>>> 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).
> 
> 
> Actually, each call to amdgpu_display_crtc_page_flip_target() will
> 
> 1, init a work(amdgpu_display_unpin_work_func) to unpin an old buffer
>    (crtc->primary->fb), the work will be queued in dce_vX_0_pageflip_irq().
> 
> 2, pin a new buffer, assign it to crtc->primary->fb. But how to unpin it?
>    Next call.
> 
> The problem is the pinned buffer of last call to 
> amdgpu_display_crtc_page_flip_target() is not unpinned.

It's unpinned in dce_v*_0_crtc_disable.


I think I've found the problem though: dce_v*_0_crtc_do_set_base pin the BO from target_fb unconditionally, but unpin the BO from the fb parameter only if it's different from the former. So if they're the same, the BO's pin count is incremented by 1.


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