linux-kernel - Re: [PATCH] drm/amd/display: fix flickering caused by S/G mode

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Message-ID: <a13a38a4-6327-7af0-56b7-930f4258eee5@amd.com>
Date:   Mon, 17 Apr 2023 17:46:00 +0200
From:   Christian König <christian.koenig@....com>
To:     Hamza Mahfooz <hamza.mahfooz@....com>,
        amd-gfx@...ts.freedesktop.org
Cc:     stable@...r.kernel.org, Harry Wentland <harry.wentland@....com>,
        Leo Li <sunpeng.li@....com>,
        Rodrigo Siqueira <Rodrigo.Siqueira@....com>,
        Alex Deucher <alexander.deucher@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Aurabindo Pillai <aurabindo.pillai@....com>,
        Qingqing Zhuo <qingqing.zhuo@....com>,
        Hans de Goede <hdegoede@...hat.com>,
        Hersen Wu <hersenxs.wu@....com>,
        Stylon Wang <stylon.wang@....com>,
        Luben Tuikov <luben.tuikov@....com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/amd/display: fix flickering caused by S/G mode

Am 17.04.23 um 17:25 schrieb Hamza Mahfooz:
>
> On 4/17/23 11:03, Christian König wrote:
>> Am 17.04.23 um 16:51 schrieb Hamza Mahfooz:
>>>
>>> On 4/17/23 01:59, Christian König wrote:
>>>> Am 14.04.23 um 21:33 schrieb Hamza Mahfooz:
>>>>> Currently, we allow the framebuffer for a given plane to move between
>>>>> memory domains, however when that happens it causes the screen to
>>>>> flicker, it is even possible for the framebuffer to change memory
>>>>> domains on every plane update (causing a continuous flicker 
>>>>> effect). So,
>>>>> to fix this, don't perform an immediate flip in the aforementioned 
>>>>> case.
>>>>
>>>> That sounds strongly like you just forget to wait for the move to 
>>>> finish!
>>>>
>>>> What is the order of things done here? E.g. who calls 
>>>> amdgpu_bo_pin() and who waits for fences for finish signaling? Is 
>>>> that maybe just in the wrong order?
>>>
>>> The pinning logic is in dm_plane_helper_prepare_fb(). Also, it seems
>>> like we wait for the fences in amdgpu_dm_atomic_commit_tail(), using
>>> drm_atomic_helper_wait_for_fences(). The ordering should be fine as
>>> well, since prepare_fb() is always called before atomic_commit_tail().
>>
>> Ok, then why is there any flickering?
>>
>> BTW reserving a fence slot is completely unnecessary. That looks like 
>> you copy&pasted the code from somewhere else without checking what it 
>> actually does.
>
> It seemed like it was necessary to read `tbo.resource` since the
> documentation for `struct ttm_buffer_object` makes mention of a
> "bo::resv::reserved" lock.

What? No, that sounds like you completely misunderstood that. I think we 
need to improve the documentation.

As long as the object is pinned for scanout you don't even need to 
reserve it.

Just use something like "abo->tbo.resource ? abo->tbo.resource->mem_type 
: 0" here.

Regards,
Christian.

>
>>
>> Regards,
>> Christian.
>>
>>>
>>>>
>>>> Regards,
>>>> Christian.
>>>>
>>>>>
>>>>> Cc: stable@...r.kernel.org
>>>>> Fixes: 81d0bcf99009 ("drm/amdgpu: make display pinning more 
>>>>> flexible (v2)")
>>>>> Signed-off-by: Hamza Mahfooz <hamza.mahfooz@....com>
>>>>> ---
>>>>>   .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 41 
>>>>> ++++++++++++++++++-
>>>>>   1 file changed, 39 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c 
>>>>> b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>>> index da3045fdcb6d..9a4e7408384a 100644
>>>>> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>>> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>>>>> @@ -7897,6 +7897,34 @@ static void amdgpu_dm_commit_cursors(struct 
>>>>> drm_atomic_state *state)
>>>>>               amdgpu_dm_plane_handle_cursor_update(plane, 
>>>>> old_plane_state);
>>>>>   }
>>>>> +static inline uint32_t get_mem_type(struct amdgpu_device *adev,
>>>>> +                    struct drm_gem_object *obj,
>>>>> +                    bool check_domain)
>>>>> +{
>>>>> +    struct amdgpu_bo *abo = gem_to_amdgpu_bo(obj);
>>>>> +    uint32_t mem_type;
>>>>> +
>>>>> +    if (unlikely(amdgpu_bo_reserve(abo, true)))
>>>>> +        return 0;
>>>>> +
>>>>> +    if (unlikely(dma_resv_reserve_fences(abo->tbo.base.resv, 1)))
>>>>> +        goto err;
>>>>> +
>>>>> +    if (check_domain &&
>>>>> +        amdgpu_display_supported_domains(adev, abo->flags) !=
>>>>> +        (AMDGPU_GEM_DOMAIN_VRAM | AMDGPU_GEM_DOMAIN_GTT))
>>>>> +        goto err;
>>>>> +
>>>>> +    mem_type = abo->tbo.resource->mem_type;
>>>>> +    amdgpu_bo_unreserve(abo);
>>>>> +
>>>>> +    return mem_type;
>>>>> +
>>>>> +err:
>>>>> +    amdgpu_bo_unreserve(abo);
>>>>> +    return 0;
>>>>> +}
>>>>> +
>>>>>   static void amdgpu_dm_commit_planes(struct drm_atomic_state *state,
>>>>>                       struct dc_state *dc_state,
>>>>>                       struct drm_device *dev,
>>>>> @@ -7916,6 +7944,7 @@ static void amdgpu_dm_commit_planes(struct 
>>>>> drm_atomic_state *state,
>>>>> to_dm_crtc_state(drm_atomic_get_old_crtc_state(state, pcrtc));
>>>>>       int planes_count = 0, vpos, hpos;
>>>>>       unsigned long flags;
>>>>> +    uint32_t mem_type;
>>>>>       u32 target_vblank, last_flip_vblank;
>>>>>       bool vrr_active = amdgpu_dm_crtc_vrr_active(acrtc_state);
>>>>>       bool cursor_update = false;
>>>>> @@ -8035,13 +8064,21 @@ static void amdgpu_dm_commit_planes(struct 
>>>>> drm_atomic_state *state,
>>>>>               }
>>>>>           }
>>>>> +        mem_type = get_mem_type(dm->adev, 
>>>>> old_plane_state->fb->obj[0],
>>>>> +                    true);
>>>>> +
>>>>>           /*
>>>>>            * Only allow immediate flips for fast updates that don't
>>>>> -         * change FB pitch, DCC state, rotation or mirroing.
>>>>> +         * change memory domain, FB pitch, DCC state, rotation or
>>>>> +         * mirroring.
>>>>>            */
>>>>> bundle->flip_addrs[planes_count].flip_immediate =
>>>>>               crtc->state->async_flip &&
>>>>> -            acrtc_state->update_type == UPDATE_TYPE_FAST;
>>>>> +            acrtc_state->update_type == UPDATE_TYPE_FAST &&
>>>>> +            (!mem_type || (mem_type && get_mem_type(dm->adev,
>>>>> +                                fb->obj[0],
>>>>> +                                false) ==
>>>>> +                       mem_type));
>>>>>           timestamp_ns = ktime_get_ns();
>>>>> bundle->flip_addrs[planes_count].flip_timestamp_in_us = 
>>>>> div_u64(timestamp_ns, 1000);
>>>>
>>