Re: [PATCH 1/2] Revert "drm/radeon: remove drm_vblank_get|put from pflip handling"

["Michel Dänzer" <michel@daenzer.net>]

On 18.06.2014 18:14, Christian König wrote:
> Am 18.06.2014 07:53, schrieb Michel Dänzer:
>>
>> Looking into these issues has got me thinking about the use of the page
>> flip interrupt: If the page flip interrupt arrives before the
>> corresponding
>> vertical blank interrupt, the DRM vblank counter will be lower than
>> expected by 1 in drm_send_vblank_event(). I suspect this is the cause of
>>
>>   (WW) RADEON(0): radeon_dri2_flip_event_handler: Pageflip completion
>> event has impossible msc [x-1] < target_msc [x]
>>
>> messages in the X log file which have been popping up in bug reports
>> lately.
>> This also results in 0s being returned to the client for the MSC and
>> timestamp of the swap completion, which could cause all kinds of bad
>> behaviour.
> First of all thanks for looking into it. Are you getting this on 3.16 or
> 3.15?

I haven't actually run into this myself yet. I thought I'd seen it in
several bug reports, but right now I can only find
https://bugs.freedesktop.org/show_bug.cgi?id=80029#c17 , which seems to
include the page flipping changes from 3.16.


> I don't think that the pflip irq is thrown earlier than the vblank, but
> on 3.16 it might actually be that we program the flip so fast into the
> hardware that we do it one frame earlier than planned.

So userspace is notified of the previous vertical blank period and calls
the page flip ioctl in response, which then manages to program the
scanout address update into the hardware before the scanout address
update is latched during the previous vertical blank period?

To avoid that scenario, one possibility might be to check if we're in
vertical blank before calling radeon_page_flip(), and if so sleep for
1ms or so before trying again? That might unnecessarily delay flips on
other CRTCs though...


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