Re: [PATCH v2 1/5] drm/rockchip: fix fb references in async update

[Helen Koike <helen.koike@collabora.com>]

On 3/15/19 8:29 AM, Michel Dänzer wrote:
> On 2019-03-15 11:25 a.m., Boris Brezillon wrote:
>> On Fri, 15 Mar 2019 11:11:36 +0100
>> Michel Dänzer <michel@daenzer.net> wrote:
>>
>>> On 2019-03-14 6:51 p.m., Helen Koike wrote:
>>>> On 3/14/19 6:15 AM, Michel Dänzer wrote:  
>>>>> On 2019-03-13 7:08 p.m., Helen Koike wrote:  
>>>>>> On 3/13/19 6:58 AM, Michel Dänzer wrote:  
>>>>>>> On 2019-03-13 4:42 a.m., Tomasz Figa wrote:  
>>>>>>>> On Wed, Mar 13, 2019 at 12:52 AM Boris Brezillon
>>>>>>>> <boris.brezillon@collabora.com> wrote:  
>>>>>>>>> On Tue, 12 Mar 2019 12:34:45 -0300
>>>>>>>>> Helen Koike <helen.koike@collabora.com> wrote:  
>>>>>>>>>> On 3/12/19 3:34 AM, Boris Brezillon wrote:  
>>>>>>>>>>> On Mon, 11 Mar 2019 23:21:59 -0300
>>>>>>>>>>> Helen Koike <helen.koike@collabora.com> wrote:
>>>>>>>>>>>  
>>>>>>>>>>>> --- a/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
>>>>>>>>>>>> +++ b/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
>>>>>>>>>>>> @@ -912,30 +912,31 @@ static void vop_plane_atomic_async_update(struct drm_plane *plane,
>>>>>>>>>>>>                                      struct drm_plane_state *new_state)
>>>>>>>>>>>>  {
>>>>>>>>>>>>    struct vop *vop = to_vop(plane->state->crtc);
>>>>>>>>>>>> -  struct drm_plane_state *plane_state;
>>>>>>>>>>>> +  struct drm_framebuffer *old_fb = plane->state->fb;
>>>>>>>>>>>>
>>>>>>>>>>>> -  plane_state = plane->funcs->atomic_duplicate_state(plane);
>>>>>>>>>>>> -  plane_state->crtc_x = new_state->crtc_x;
>>>>>>>>>>>> -  plane_state->crtc_y = new_state->crtc_y;
>>>>>>>>>>>> -  plane_state->crtc_h = new_state->crtc_h;
>>>>>>>>>>>> -  plane_state->crtc_w = new_state->crtc_w;
>>>>>>>>>>>> -  plane_state->src_x = new_state->src_x;
>>>>>>>>>>>> -  plane_state->src_y = new_state->src_y;
>>>>>>>>>>>> -  plane_state->src_h = new_state->src_h;
>>>>>>>>>>>> -  plane_state->src_w = new_state->src_w;
>>>>>>>>>>>> -
>>>>>>>>>>>> -  if (plane_state->fb != new_state->fb)
>>>>>>>>>>>> -          drm_atomic_set_fb_for_plane(plane_state, new_state->fb);
>>>>>>>>>>>> -
>>>>>>>>>>>> -  swap(plane_state, plane->state);
>>>>>>>>>>>> -
>>>>>>>>>>>> -  if (plane->state->fb && plane->state->fb != new_state->fb) {
>>>>>>>>>>>> +  /*
>>>>>>>>>>>> +   * A scanout can still be occurring, so we can't drop the reference to
>>>>>>>>>>>> +   * the old framebuffer. To solve this we get a reference to old_fb and
>>>>>>>>>>>> +   * set a worker to release it later.  
>>>>>>>>>>>
>>>>>>>>>>> Hm, doesn't look like an async update to me if we have to wait for the
>>>>>>>>>>> next VBLANK to happen to get the new content on the screen. Maybe we
>>>>>>>>>>> should reject async updates when old_fb != new_fb in the rk  
>>>>>>>>>>> ->async_check() hook.  
>>>>>>>>>>
>>>>>>>>>> Unless I am misunderstanding this, we don't wait here, we just grab a
>>>>>>>>>> reference to the fb in case it is being still used by the hw, so it
>>>>>>>>>> doesn't get released prematurely.  
>>>>>>>>>
>>>>>>>>> I was just reacting to the comment that says the new FB should stay
>>>>>>>>> around until the next VBLANK event happens. If the FB must stay around
>>>>>>>>> that probably means the HW is still using, which made me wonder if this
>>>>>>>>> HW actually supports async update (where async means "update now and
>>>>>>>>> don't care about about tearing"). Or maybe it takes some time to switch
>>>>>>>>> to the new FB and waiting for the next VBLANK to release the old FB was
>>>>>>>>> an easy solution to not wait for the flip to actually happen in  
>>>>>>>>> ->async_update() (which is kind of a combination of async+non-blocking).  
>>>>>>>>
>>>>>>>> The hardware switches framebuffers on vblank, so whatever framebuffer
>>>>>>>> is currently being scanned out from needs to stay there until the
>>>>>>>> hardware switches to the new one in shadow registers. If that doesn't
>>>>>>>> happen, you get IOMMU faults and the display controller stops working
>>>>>>>> since we don't have any fault handling currently, just printing a
>>>>>>>> message.  
>>>>>>>
>>>>>>> Sounds like your hardware doesn't actually support async flips. It's
>>>>>>> probably better for the driver not to pretend otherwise.  
>>>>>>
>>>>>> I think wee need to clarify the meaning of the async_update callback
>>>>>> (and we should clarify it in the docs).
>>>>>>
>>>>>> The way I understand what the async_update callback should do is: don't
>>>>>> block (i.e. don't wait for the next vblank),  
>>>>>
>>>>> Note that those are two separate things. "Async flips" are about "don't
>>>>> wait for vblank", not about "don't block".
>>>>>
>>>>>  
>>>>>> and update the hw state at some point with the latest state from the
>>>>>> last call to async_update.
>>>>>>
>>>>>> Which means that: any driver can implement the async_update callback,
>>>>>> independently if it supports changing its state right away or not.
>>>>>> If hw supports, async_update can change the hw state right away, if not,
>>>>>> then changes will be applied in the next vblank (it can even amend the
>>>>>> pending commit if there is one).
>>>>>> With this, we can remove all the legacy cursor code to use the
>>>>>> async_update callback, since async_update can be called 100 times before
>>>>>> the next vblank, and the latest state will be set to the hw without
>>>>>> waiting 100 vblanks.
>>>>>>
>>>>>> Please, let me know if this is your understanding as well. If not, then
>>>>>> we need to remodel things.  
>>>>>
>>>>> While this may make sense for cursor updates, I don't think it does for
>>>>> async flips. If the flip only actually takes effect during the next
>>>>> vblank, it doesn't really fit the definition and userspace expectation
>>>>> of an async flip. It's better to clearly communicate to userspace that
>>>>> the hardware cannot do async flips, than to pretend it can and fake
>>>>> them. Userspace has to deal with this anyway, since async flips weren't
>>>>> always supported in general.  
>>>>
>>>> What do you think if we separate two concepts here:
>>>>
>>>> - amend mode: works like cursor updates, i.e, update the hw state at
>>>> some point with the latest state from the last call to async_update. No
>>>> special hardware support is required.
>>>>
>>>> - async update: update hw state immediately. This depends if the hw
>>>> supports it or not.
>>>>
>>>> Every async update is an amend, but the opposite is not necessarily true.
>>>>
>>>> What do you think if we rename the current async_update to amend_update,
>>>> and we add a parameter "force_async" to it? (or maybe
>>>> force_immediate_update?)
>>>> Then amend_check with force_async=1 would fail if the hardware doesn't
>>>> support it (we could also add flags in the capabilities to inform
>>>> userspace the expected behaviour of things and if the hw supports
>>>> force_sync).
>>>>
>>>> Like this, we can implement the cursors using the amend_update (which is
>>>> now called async_update), and async_flips with amend_update with
>>>> force_async=1.  
>>>
>>> Might force_async make sense for cursor updates as well? I thought some
>>> hardware supported HW cursor updates outside of vblank, but I'm not sure.

What I had in mind was actually:
amend_update() -> could do a real async or not depending on the hw
force_async=1 -> it means amend_update will fail if the hw doesn't
support it.

>>>
>>> Without force_async, are cursor updates always applied to the hardware
>>> on the next vblank, even if the pending commit is delayed further (e.g.
>>> because a fence it depends on doesn't signal before vblank)? If cursor
>>> updates can be delayed beyond the next vblank, that can result in bad
>>> user experience.
>>
>> You mean you have
>>
>> 1. sync/regular update pending (waiting on a fence)
>> 2. async update on top of #1
>>
>> ?
> 
> Yeah.
> 
> 

Actually I was thinking in another solution (without this force_async flag).

Instead of having this force_async, we can have two capabilities:

CAP_ASYNC: means the hw supports real async
CAP_AMEND: means that the driver supports amend the in-flight update so
that the new one will take its place in the queue (i.e. the current
legacy cursor behavior).

If (!CAP_AMEND && !CAP_ASYNC)
	* use a sync update or update the FB content in place without flipping
buffers.
	* legacy cursor update will fallback to sync update.
	* async flip is not supported.

If (CAP_AMEND && !CAP_ASYNC)
	* legacy cursor update will amend in-flight pending updates (like how
rockchip does now) or it will fallback to a sync update if not possible.
	* async flip is not supported.

If (!CAP_AMEND && CAP_ASYNC)
	* not sure yet what this would mean.

If (CAP_AMEND && CAP_ASYNC)
	* legacy cursor update will perform real async update.
	* async flip is supported.


What do you think?

Regards
Helen