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Date:   Fri, 15 Mar 2019 12:29:45 +0100
From:   Michel Dänzer <michel@...nzer.net>
To:     Boris Brezillon <boris.brezillon@...labora.com>
Cc:     Stéphane Marchesin <marcheu@...gle.com>,
        Sean Paul <seanpaul@...gle.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Tomasz Figa <tfiga@...omium.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        Helen Koike <helen.koike@...labora.com>,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        kernel@...labora.com, nicholas.kazlauskas@....com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/5] drm/rockchip: fix fb references in async update

On 2019-03-15 11:25 a.m., Boris Brezillon wrote:
> On Fri, 15 Mar 2019 11:11:36 +0100
> Michel Dänzer <michel@...nzer.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@...labora.com> wrote:  
>>>>>>>> On Tue, 12 Mar 2019 12:34:45 -0300
>>>>>>>> Helen Koike <helen.koike@...labora.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@...labora.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.
>>
>> 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.


-- 
Earthling Michel Dänzer               |              https://www.amd.com
Libre software enthusiast             |             Mesa and X developer

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