Re: [PATCH] drm/vc4: Return -EBUSY if there's already a pending flip event.

[Eric Anholt]

Robert Foss <robert.foss@xxxxxxxxxxxxx> writes:

> On 05/02/2016 08:57 PM, Eric Anholt wrote:
>> robert.foss@xxxxxxxxxxxxx writes:
>>
>>> From: Robert Foss <robert.foss@xxxxxxxxxxxxx>
>>>
>>> As per the docs, atomic_commit should return -EBUSY "if an asycnhronous
>>> update is requested and there is an earlier update pending".
>>
>> Note: docs cited here are drm_crtc.h, and the whole quote is:
>>
>> 	 *  - -EBUSY, if an asynchronous updated is requested and there is
>> 	 *    an earlier updated pending. Drivers are allowed to support a queue
>> 	 *    of outstanding updates, but currently no driver supports that.
>> 	 *    Note that drivers must wait for preceding updates to complete if a
>> 	 *    synchronous update is requested, they are not allowed to fail the
>> 	 *    commit in that case.
>>
>>> Signed-off-by: Robert Foss <robert.foss@xxxxxxxxxxxxx>
>>> ---
>>>   drivers/gpu/drm/vc4/vc4_crtc.c |  6 ++++++
>>>   drivers/gpu/drm/vc4/vc4_drv.h  |  1 +
>>>   drivers/gpu/drm/vc4/vc4_kms.c  | 20 ++++++++++++++++++--
>>>   3 files changed, 25 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/vc4/vc4_crtc.c b/drivers/gpu/drm/vc4/vc4_crtc.c
>>> index 355ee4b..43193a3 100644
>>> --- a/drivers/gpu/drm/vc4/vc4_crtc.c
>>> +++ b/drivers/gpu/drm/vc4/vc4_crtc.c
>>> @@ -802,3 +802,9 @@ struct platform_driver vc4_crtc_driver = {
>>>   		.of_match_table = vc4_crtc_dt_match,
>>>   	},
>>>   };
>>> +
>>> +bool vc4_crtc_has_pending_event(struct drm_crtc *crtc)
>>> +{
>>> +	assert_spin_locked(&crtc->dev->event_lock);
>>> +	return to_vc4_crtc(crtc)->event;
>>> +}
>>> diff --git a/drivers/gpu/drm/vc4/vc4_drv.h b/drivers/gpu/drm/vc4/vc4_drv.h
>>> index fa2ad15..54c1fb5 100644
>>> --- a/drivers/gpu/drm/vc4/vc4_drv.h
>>> +++ b/drivers/gpu/drm/vc4/vc4_drv.h
>>> @@ -414,6 +414,7 @@ extern struct platform_driver vc4_crtc_driver;
>>>   int vc4_enable_vblank(struct drm_device *dev, unsigned int crtc_id);
>>>   void vc4_disable_vblank(struct drm_device *dev, unsigned int crtc_id);
>>>   int vc4_crtc_debugfs_regs(struct seq_file *m, void *arg);
>>> +bool vc4_crtc_has_pending_event(struct drm_crtc *crtc);
>>>
>>>   /* vc4_debugfs.c */
>>>   int vc4_debugfs_init(struct drm_minor *minor);
>>> diff --git a/drivers/gpu/drm/vc4/vc4_kms.c b/drivers/gpu/drm/vc4/vc4_kms.c
>>> index 4718ae5..dc157a1e 100644
>>> --- a/drivers/gpu/drm/vc4/vc4_kms.c
>>> +++ b/drivers/gpu/drm/vc4/vc4_kms.c
>>> @@ -107,10 +107,26 @@ static int vc4_atomic_commit(struct drm_device *dev,
>>>   			     bool async)
>>>   {
>>>   	struct vc4_dev *vc4 = to_vc4_dev(dev);
>>> -	int ret;
>>> -	int i;
>>>   	uint64_t wait_seqno = 0;
>>>   	struct vc4_commit *c;
>>> +	struct drm_crtc_state *crtc_state;
>>> +	struct drm_crtc *crtc;
>>> +	unsigned long flags;
>>> +	int i, ret;
>>> +
>>> +	if (async) {
>>> +		for_each_crtc_in_state(state, crtc, crtc_state, i) {
>>> +
>>> +			spin_lock_irqsave(&dev->event_lock, flags);
>>> +
>>> +			if (crtc->state->event ||
>>> +				vc4_crtc_has_pending_event(crtc)) {
>>> +				spin_unlock_irqrestore(&dev->event_lock, flags);
>>> +				return -EBUSY;
>>> +			}
>>> +			spin_unlock_irqrestore(&dev->event_lock, flags);
>>> +		}
>>> +	}
>>
>> By my reading, the point of returning -EBUSY here is just to avoid
>> blocking when the compositor asked for nonblocking.  You're checking
>> that each CRTC involved in the commit doesn't have a queued pageflip
>> event, except that then we immediately proceed to:
>>
>> 	/* Make sure that any outstanding modesets have finished. */
>> 	ret = down_interruptible(&vc4->async_modeset);
>> 	if (ret) {
>> 		kfree(c);
>> 		return ret;
>> 	}
>>
>> so this per-CRTC check doesn't prevent blocking if the set of CRTCs for
>> this commit was disjoint from the last one, right?  To implement the
>> minimal behavior, I think we'd want to just down_trylock instead in the
>> async case, I think.  And to implement the disjoint CRTC thing you were
>> trying for, I think we'd need to track a mask kind of like msm's
>> pending_crtcs.
>>
>
> So what you're saying is that the set of CRTCs in state
> might not contain all CRTCs and that this check might be incomplete for 
> that reason?
>
> I'm fairly new to these waters, so don't hesitate to tell me if I seem
> to be misunderstanding something or am on a wild goose chase of some sort.
>
> So you're suggesting something like this instead of
> the per CRTC check:
>
> - 	/* Make sure that any outstanding modesets have finished. */
> - 	ret = down_interruptible(&vc4->async_modeset);
> - 	if (ret) {
> - 		kfree(c);
> - 		return ret;
> - 	}
> + 	/* Make sure that any outstanding modesets have finished. */
> + 	ret = down_trylock(&vc4->async_modeset);
> + 	if (ret) {
> + 		kfree(c);
> + 		return -EBUSY;
> + 	}
>
> I've quickly tested the above patch and it does seem to work and fulfill 
> all of my requirements.

Note that you'd want to trylock only in the async case.  In the sync
case, you'd still want to block.

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