Date:   Mon, 11 Mar 2019 13:15:23 +0000
From:   "Kazlauskas, Nicholas" <Nicholas.Kazlauskas@....com>
To:     Boris Brezillon <boris.brezillon@...labora.com>
CC:     Helen Koike <helen.koike@...labora.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "Grodzovsky, Andrey" <Andrey.Grodzovsky@....com>,
        "daniel.vetter@...ll.ch" <daniel.vetter@...ll.ch>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Tomasz Figa <tfiga@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        Sean Paul <seanpaul@...gle.com>,
        "kernel@...labora.com" <kernel@...labora.com>,
        "Wentland, Harry" <Harry.Wentland@....com>,
        Stéphane Marchesin <marcheu@...gle.com>,
        Eric Anholt <eric@...olt.net>
Subject: Re: [PATCH 1/5] drm: don't block fb changes for async plane updates

On 3/11/19 6:06 AM, Boris Brezillon wrote:
> Hello Nicholas,
> 
> On Mon, 4 Mar 2019 15:46:49 +0000
> "Kazlauskas, Nicholas" <Nicholas.Kazlauskas@....com> wrote:
> 
>> On 3/4/19 9:49 AM, Helen Koike wrote:
>>> In the case of a normal sync update, the preparation of framebuffers (be
>>> it calling drm_atomic_helper_prepare_planes() or doing setups with
>>> drm_framebuffer_get()) are performed in the new_state and the respective
>>> cleanups are performed in the old_state.
>>>
>>> In the case of async updates, the preparation is also done in the
>>> new_state but the cleanups are done in the new_state (because updates
>>> are performed in place, i.e. in the current state).
>>>
>>> The current code blocks async udpates when the fb is changed, turning
>>> async updates into sync updates, slowing down cursor updates and
>>> introducing regressions in igt tests with errors of type:
>>>
>>> "CRITICAL: completed 97 cursor updated in a period of 30 flips, we
>>> expect to complete approximately 15360 updates, with the threshold set
>>> at 7680"
>>>
>>> Fb changes in async updates were prevented to avoid the following scenario:
>>>
>>> - Async update, oldfb = NULL, newfb = fb1, prepare fb1, cleanup fb1
>>> - Async update, oldfb = fb1, newfb = fb2, prepare fb2, cleanup fb2
>>> - Non-async commit, oldfb = fb2, newfb = fb1, prepare fb1, cleanup fb2 (wrong)
>>> Where we have a single call to prepare fb2 but double cleanup call to fb2.
>>>
>>> To solve the above problems, instead of blocking async fb changes, we
>>> place the old framebuffer in the new_state object, so when the code
>>> performs cleanups in the new_state it will cleanup the old_fb and we
>>> will have the following scenario instead:
>>>
>>> - Async update, oldfb = NULL, newfb = fb1, prepare fb1, no cleanup
>>> - Async update, oldfb = fb1, newfb = fb2, prepare fb2, cleanup fb1
>>> - Non-async commit, oldfb = fb2, newfb = fb1, prepare fb1, cleanup fb2
>>>
>>> Where calls to prepare/cleanup are ballanced.
>>>
>>> Cc: <stable@...r.kernel.org> # v4.14+: 25dc194b34dd: drm: Block fb changes for async plane updates
>>> Fixes: 25dc194b34dd ("drm: Block fb changes for async plane updates")
>>> Suggested-by: Boris Brezillon <boris.brezillon@...labora.com>
>>> Signed-off-by: Helen Koike <helen.koike@...labora.com>
>>>
>>> ---
>>> Hello,
>>>
>>> As mentioned in the cover letter,
>>> I tested on the rockchip and on i915 (with a patch I am still working on for
>>> replacing cursors by async update), with igt plane_cursor_legacy and
>>> kms_cursor_legacy and I didn't see any regressions.
>>> I couldn't test on MSM and AMD because I don't have the hardware (and I am
>>> having some issues testing on vc4) and I would appreciate if anyone could help
>>> me testing those.
>>>
>>> I also think it would be a better solution if, instead of having async
>>> to do in-place updates in the current state, the async path should be
>>> equivalent to a syncronous update, i.e., modifying new_state and
>>> performing a flip
>>> IMHO, the only difference between sync and async should be that async update
>>> doesn't wait for vblank and applies the changes immeditally to the hw,
>>> but the code path could be almost the same.
>>> But for now I think this solution is ok (swaping new_fb/old_fb), and
>>> then we can adjust things little by little, what do you think?
>>>
>>> Thanks!
>>> Helen
>>>
>>>    drivers/gpu/drm/drm_atomic_helper.c | 20 ++++++++++----------
>>>    1 file changed, 10 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
>>> index 540a77a2ade9..e7eb96f1efc2 100644
>>> --- a/drivers/gpu/drm/drm_atomic_helper.c
>>> +++ b/drivers/gpu/drm/drm_atomic_helper.c
>>> @@ -1608,15 +1608,6 @@ int drm_atomic_helper_async_check(struct drm_device *dev,
>>>    	    old_plane_state->crtc != new_plane_state->crtc)
>>>    		return -EINVAL;
>>>    
>>> -	/*
>>> -	 * FIXME: Since prepare_fb and cleanup_fb are always called on
>>> -	 * the new_plane_state for async updates we need to block framebuffer
>>> -	 * changes. This prevents use of a fb that's been cleaned up and
>>> -	 * double cleanups from occuring.
>>> -	 */
>>> -	if (old_plane_state->fb != new_plane_state->fb)
>>> -		return -EINVAL;
>>> -
>>>    	funcs = plane->helper_private;
>>>    	if (!funcs->atomic_async_update)
>>>    		return -EINVAL;
>>> @@ -1657,6 +1648,9 @@ void drm_atomic_helper_async_commit(struct drm_device *dev,
>>>    	int i;
>>>    
>>>    	for_each_new_plane_in_state(state, plane, plane_state, i) {
>>> +		struct drm_framebuffer *new_fb = plane_state->fb;
>>> +		struct drm_framebuffer *old_fb = plane->state->fb;
>>> +
>>>    		funcs = plane->helper_private;
>>>    		funcs->atomic_async_update(plane, plane_state);
>>>    
>>> @@ -1665,11 +1659,17 @@ void drm_atomic_helper_async_commit(struct drm_device *dev,
>>>    		 * plane->state in-place, make sure at least common
>>>    		 * properties have been properly updated.
>>>    		 */
>>> -		WARN_ON_ONCE(plane->state->fb != plane_state->fb);
>>> +		WARN_ON_ONCE(plane->state->fb != new_fb);
>>>    		WARN_ON_ONCE(plane->state->crtc_x != plane_state->crtc_x);
>>>    		WARN_ON_ONCE(plane->state->crtc_y != plane_state->crtc_y);
>>>    		WARN_ON_ONCE(plane->state->src_x != plane_state->src_x);
>>>    		WARN_ON_ONCE(plane->state->src_y != plane_state->src_y);
>>> +
>>> +		/*
>>> +		 * Make sure the FBs have been swapped so that cleanups in the
>>> +		 * new_state performs a cleanup in the old FB.
>>> +		 */
>>> +		WARN_ON_ONCE(plane_state->fb != old_fb);
>>
>> I personally think this approach is fine and the WARN_ON s are good for
>> catching drivers that want to use these in the future.
> 
> Well, I agree this change is the way to go for a short-term solution
> to relax the old_fb == new_fb constraint, but I keep thinking this whole
> "update plane_state in place" is a recipe for trouble and just make
> things more complicated for drivers for no obvious reasons. Look at the
> VC4 implem [1] if you need a proof that things can get messy pretty
> quickly.
> 
> All this state-fields-copying steps could be skipped if the core was
> simply swapping the old/new states as is done in the sync update path.
> 
> [1]https://elixir.bootlin.com/linux/v5.0-rc7/source/drivers/gpu/drm/vc4/vc4_plane.c#L878

I completely agree with this view FWIW. I had a discussion with Daniel 
about this when I had posted the original block FB changes patch.

- The plane object needs to be locked in order for async state to be updated
- Blocking commit work holds the lock for the plane, async update won't 
happen
- Non-blocking commit work that's still ongoing won't have hw_done 
signaled and drm_atomic_helper_async_check will block the async update

So this looks safe in theory, with the exception of the call to 
drm_atomic_helper_cleanup_planes occuring after hw_done is signaled.

I believe that the behavior of this function still remains the same even 
if plane->state is swapped to something else during the call (since 
old_plane_state should never be equal to plane->state if the commit 
succeeded and the plane is in the commit), but I'm not sure that's 
something we'd want to rely on.

I think other than that issue, you could probably just:

drm_atomic_helper_prepare_planes(...);
drm_atomic_helper_swap_state(...);
drm_atomic_state_get(state);
drm_atomic_helper_async_commit(...);
drm_atomic_helper_cleanup_planes(dev, state);

and it would work as expected. But there still may be other things I'm 
missing or haven't considered here.

Nicholas Kazlauskas

> 
>>
>> I do think it would be good to add something to the function docs that
>> explains this requirement and the issue that it addresses. It's a little
>> unintuitive to require that the old fb is placed into the new state, but
>> it makes sense as a workaround to this problem.
>>
>> Nicholas Kazlauskas
>>
>>>    	}
>>>    }
>>>    EXPORT_SYMBOL(drm_atomic_helper_async_commit);
>>>    
>>
> 

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