Re: [PATCH v2 5/5] drm: don't block fb changes for async plane updates

["Kazlauskas, Nicholas" <Nicholas.Kazlauskas@amd.com>]

On 3/12/19 2:44 AM, Boris Brezillon wrote:
> On Mon, 11 Mar 2019 23:22:03 -0300
> Helen Koike <helen.koike@collabora.com> 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 balanced.
>>
>> Cc: <stable@vger.kernel.org> # v4.14+
>> Fixes: 25dc194b34dd ("drm: Block fb changes for async plane updates")
>> Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
>> Signed-off-by: Helen Koike <helen.koike@collabora.com>
> 
> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>

I was thinking that the comment could go in async_commit or async_check, 
but I guess it works there too. Maybe it needs a FIXME or a TODO for a 
full state swap, but these are just nitpicks.

Nicholas Kazlauskas

> 
>>
>> ---
>> Hello,
>>
>> As mentioned in the cover letter, I tested in almost all platforms with
>> igt plane_cursor_legacy and kms_cursor_legacy and I didn't see any
>> regressions. But I couldn't test on MSM and AMD because I don't have
>> the hardware I would appreciate if anyone could help me testing those.
>>
>> Thanks!
>> Helen
>>
>> Changes in v2:
>> - Change the order of the patch in the series, add this as the last one.
>> - Add documentation
>> - s/ballanced/balanced
>>
>>   drivers/gpu/drm/drm_atomic_helper.c      | 20 ++++++++++----------
>>   include/drm/drm_modeset_helper_vtables.h |  5 +++++
>>   2 files changed, 15 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);
>>   	}
>>   }
>>   EXPORT_SYMBOL(drm_atomic_helper_async_commit);
>> diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
>> index cfb7be40bed7..ce582e8e8f2f 100644
>> --- a/include/drm/drm_modeset_helper_vtables.h
>> +++ b/include/drm/drm_modeset_helper_vtables.h
>> @@ -1174,6 +1174,11 @@ struct drm_plane_helper_funcs {
>>   	 * current one with the new plane configurations in the new
>>   	 * plane_state.
>>   	 *
>> +	 * Drivers should also swap the framebuffers between plane state
>> +	 * and new_state. This is required because prepare and cleanup calls
>> +	 * are performed on the new_state object, then to cleanup the old
>> +	 * framebuffer, it needs to be placed inside the new_state object.
>> +	 *
>>   	 * FIXME:
>>   	 *  - It only works for single plane updates
>>   	 *  - Async Pageflips are not supported yet
>