Re: [PATCH v3 3/5] drm/msm: fix fb references in async update

[Helen Koike]

Hi Rob,

On 3/27/19 11:52 AM, Rob Clark wrote:
> On Wed, Mar 13, 2019 at 8:21 PM Helen Koike <helen.koike@xxxxxxxxxxxxx> wrote:
>>
>> Async update callbacks are expected to set the old_fb in the new_state
>> so prepare/cleanup framebuffers are balanced.
>>
>> Cc: <stable@xxxxxxxxxxxxxxx> # v4.14+
>> Fixes: 224a4c970987 ("drm/msm: update cursors asynchronously through atomic")
>> Suggested-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
>> Signed-off-by: Helen Koike <helen.koike@xxxxxxxxxxxxx>
>>
>> ---
>> Hello,
>>
>> As mentioned in the cover letter,
>> But I couldn't test on MSM because I don't have the hardware and I would
>> appreciate if anyone could test it.
>>
>> In other platforms (VC4, AMD, Rockchip), there is a hidden
>> drm_framebuffer_get(new_fb)/drm_framebuffer_put(old_fb) in async_update
>> that is wrong, but I couldn't identify those here, not sure if it is hidden
>> somewhere else, but if tests fail this is probably the cause.
>>
>> Thanks!
>> Helen
> 
> Apologies, I haven't had a chance to test this yet, and it is likely
> to be a few weeks before I have a chance..
> 
> That said, not refcnt'ing the fb seems suspicious.   I read the
> explanation in the other patches, but kind of think it is more clear
> to keep the extra refcnt'ing.. or at least have a comment.


Refcnt'ing is taken care by drm_mode_cursor_universal() when calling
drm_atomic_helper_update_plane().
The other codes had a drm_atomic_set_fb_for_plane() (which get a
referent of the new fb and put the old fb), but the problem is that the
cleanup_plane will also put the ref of the old fb.
In the other drivers I saw, drm_atomic_set_fb_for_plane() shouldn't be
called, but this issue should be triggered very easily with a test.

I can add this is in the async_check() docs.

> 
> I do a bit wonder if we hold the ref to the outgoing cursor long
> enough.. I think the hw will actually continue to scan it out until
> the next vblank.  But Archit was the one who added async_update
> support so maybe that is handled somewhere else.  It's been a while
> since I've looked at this.

I don't know much about MSM, but if I understand correctly, MSM don't
support true async, is that correct?

Then I think we need to do something similar to the rockchip:

https://patchwork.kernel.org/patch/10852039/

and setup a drm_flip_work_queue().
But this seems another problem that is already there no? Not really
related to this patch. I could send a separate patch for this (but I
don't have the means to test it).

Regards,
Helen

> 
> BR,
> -R
> 
> 
> 
>>
>> Changes in v3: None
>> Changes in v2:
>> - update CC stable and Fixes tag
>>
>>  drivers/gpu/drm/msm/disp/mdp5/mdp5_plane.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/msm/disp/mdp5/mdp5_plane.c b/drivers/gpu/drm/msm/disp/mdp5/mdp5_plane.c
>> index be13140967b4..b854f471e9e5 100644
>> --- a/drivers/gpu/drm/msm/disp/mdp5/mdp5_plane.c
>> +++ b/drivers/gpu/drm/msm/disp/mdp5/mdp5_plane.c
>> @@ -502,6 +502,8 @@ static int mdp5_plane_atomic_async_check(struct drm_plane *plane,
>>  static void mdp5_plane_atomic_async_update(struct drm_plane *plane,
>>                                            struct drm_plane_state *new_state)
>>  {
>> +       struct drm_framebuffer *old_fb = plane->state->fb;
>> +
>>         plane->state->src_x = new_state->src_x;
>>         plane->state->src_y = new_state->src_y;
>>         plane->state->crtc_x = new_state->crtc_x;
>> @@ -524,6 +526,8 @@ static void mdp5_plane_atomic_async_update(struct drm_plane *plane,
>>
>>         *to_mdp5_plane_state(plane->state) =
>>                 *to_mdp5_plane_state(new_state);
>> +
>> +       new_state->fb = old_fb;
>>  }
>>
>>  static const struct drm_plane_helper_funcs mdp5_plane_helper_funcs = {
>> --
>> 2.20.1
>>
>