Date:   Wed, 13 Mar 2019 10:58:25 +0100
From:   Michel Dänzer <michel@...nzer.net>
To:     Tomasz Figa <tfiga@...omium.org>,
        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>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        Helen Koike <helen.koike@...labora.com>, kernel@...labora.com,
        nicholas.kazlauskas@....com,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/5] drm/rockchip: fix fb references in async update

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:
>>>>
>>>>> In the case of async update, modifications are done in place, i.e. in the
>>>>> current plane state, so the new_state is prepared and the new_state is
>>>>> cleanup up (instead of the old_state, diferrently on what happen in a
>>>>
>>>>   ^ cleaned up                              ^ differently (but maybe
>>>> "unlike what happens" is more appropriate here).
>>>>
>>>>> normal sync update).
>>>>> To cleanup the old_fb properly, it needs to be placed in the new_state
>>>>> in the end of async_update, so cleanup call will unreference the old_fb
>>>>> correctly.
>>>>>
>>>>> Also, the previous code had a:
>>>>>
>>>>>    plane_state = plane->funcs->atomic_duplicate_state(plane);
>>>>>    ...
>>>>>    swap(plane_state, plane->state);
>>>>>
>>>>>    if (plane->state->fb && plane->state->fb != new_state->fb) {
>>>>>    ...
>>>>>    }
>>>>>
>>>>> Which was wrong, as the fb were just assigned to be equal, so this if
>>>>> statement nevers evaluates to true.
>>>>>
>>>>> Another details is that the function drm_crtc_vblank_get() can only be
>>>>> called when vop->is_enabled is true, otherwise it has no effect and
>>>>> trows a WARN_ON().
>>>>>
>>>>> Calling drm_atomic_set_fb_for_plane() (which get a referent of the new
>>>>> fb and pus the old fb) is not required, as it is taken care by
>>>>> drm_mode_cursor_universal() when calling
>>>>> drm_atomic_helper_update_plane().
>>>>>
>>>>> Signed-off-by: Helen Koike <helen.koike@...labora.com>
>>>>>
>>>>> ---
>>>>> Hello,
>>>>>
>>>>> I tested on the rockchip ficus v1.1 using igt plane_cursor_legacy and
>>>>> kms_cursor_legacy and I didn't see any regressions.
>>>>>
>>>>> Changes in v2: None
>>>>>
>>>>>  drivers/gpu/drm/rockchip/rockchip_drm_vop.c | 42 ++++++++++++---------
>>>>>  1 file changed, 24 insertions(+), 18 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/rockchip/rockchip_drm_vop.c b/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
>>>>> index c7d4c6073ea5..a1ee8c156a7b 100644
>>>>> --- 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.


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

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