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Date:   Tue, 14 Jul 2020 07:20:09 -0300
From:   Melissa Wen <melissa.srw@gmail.com>
To:     Sidong Yang <realwakka@gmail.com>,
        Haneen Mohammed <hamohammed.sa@gmail.com>,
        Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>,
        David Airlie <airlied@linux.ie>,
        Emil Velikov <emil.l.velikov@gmail.com>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org
Subject: Re: [PATCH] drm/vkms: add wait_for_vblanks in atomic_commit_tail
Message-ID: <20200714102009.4qxgdq5bkpwhhhio@smtp.gmail.com>
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 <20200713160621.GQ3278063@phenom.ffwll.local>
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On 07/13, Daniel Vetter wrote:
> On Fri, Jul 10, 2020 at 02:05:33PM -0300, Melissa Wen wrote:
> > On 07/02, Daniel Vetter wrote:
> > > On Wed, Jul 01, 2020 at 03:31:34PM +0000, Sidong Yang wrote:
> > > > there is an error when igt test is run continuously. vkms_atomic_commit_tail()
> > > > need to call drm_atomic_helper_wait_for_vblanks() for give up ownership of
> > > > vblank events. without this code, next atomic commit will not enable vblank
> > > > and raise timeout error.
> > > > 
> > > > Signed-off-by: Sidong Yang <realwakka@gmail.com>
> > > > ---
> > > >  drivers/gpu/drm/vkms/vkms_drv.c | 2 ++
> > > >  1 file changed, 2 insertions(+)
> > > > 
> > > > diff --git a/drivers/gpu/drm/vkms/vkms_drv.c b/drivers/gpu/drm/vkms/vkms_drv.c
> > > > index 1e8b2169d834..10b9be67a068 100644
> > > > --- a/drivers/gpu/drm/vkms/vkms_drv.c
> > > > +++ b/drivers/gpu/drm/vkms/vkms_drv.c
> > > > @@ -93,6 +93,8 @@ static void vkms_atomic_commit_tail(struct drm_atomic_state *old_state)
> > > >  		flush_work(&vkms_state->composer_work);
> > > >  	}
> > > >  
> > > > +	drm_atomic_helper_wait_for_vblanks(dev, old_state);
> > > 
> > > Uh, we have a wait_for_flip_done right above, which should be doing
> > > exactly the same, but more precisely: Instead of just waiting for any
> > > vblank to happen, we wait for exactly the vblank corresponding to this
> > > atomic commit. So no races possible. If this is papering over some issue,
> > > then I think more debugging is needed.
> > > 
> > > What exactly is going wrong here for you?
> > 
> > Hi Daniel and Sidong,
> > 
> > I noticed a similar issue when running the IGT test kms_cursor_crc. For
> > example, a subtest that passes on the first run (alpha-opaque) fails on
> > the second due to a kind of busy waiting in subtest preparation (the
> > subtest fails before actually running).
> > 
> > In addition, in the same test, the dpms subtest started to fail since
> > the commit that change from wait_for_vblanks to wait_for_flip_done. By
> > reverting this commit, the dpms subtest passes again and the sequential
> > subtests return to normal.
> > 
> > I am trying to figure out what's missing from using flip_done op on
> > vkms, since I am also interested in solving this problem and I
> > understand that the change for flip_done has been discussed in the past.
> > 
> > Do you have any idea?
> 
> Uh, not at all. This is indeed rather surprising ...
> 
> What exactly is the failure mode when running a test the 2nd time? Full
> igt logs might give me an idea. But yeah this is kinda surprising.

Hi Daniel,

This is the IGT log of the 2nd run of kms_cursor_crc/alpha-opaque:

IGT-Version: 1.25-NO-GIT (x86_64) (Linux: 5.8.0-rc2-DRM+ x86_64)
Force option used: Using driver vkms
Starting subtest: pipe-A-cursor-alpha-opaque
Timed out: Opening crc fd, and poll for first CRC.
Subtest pipe-A-cursor-alpha-opaque failed.
**** DEBUG ****
(kms_cursor_crc:2317) igt_kms-DEBUG: display: Virtual-1: set_pipe(A)
(kms_cursor_crc:2317) igt_kms-DEBUG: display: Virtual-1: Selecting pipe A
(kms_cursor_crc:2317) igt_fb-DEBUG: igt_create_fb_with_bo_size(width=1024, height=768, format=XR24(0x34325258), modifier=0x0, size=0)
(kms_cursor_crc:2317) igt_fb-DEBUG: igt_create_fb_with_bo_size(handle=1, pitch=4096)
(kms_cursor_crc:2317) igt_fb-DEBUG: Test requirement passed: cairo_surface_status(fb->cairo_surface) == CAIRO_STATUS_SUCCESS
(kms_cursor_crc:2317) igt_fb-DEBUG: igt_create_fb_with_bo_size(width=1024, height=768, format=XR24(0x34325258), modifier=0x0, size=0)
(kms_cursor_crc:2317) igt_fb-DEBUG: igt_create_fb_with_bo_size(handle=2, pitch=4096)
(kms_cursor_crc:2317) igt_fb-DEBUG: Test requirement passed: cairo_surface_status(fb->cairo_surface) == CAIRO_STATUS_SUCCESS
(kms_cursor_crc:2317) igt_kms-DEBUG: Test requirement passed: plane_idx >= 0 && plane_idx < pipe->n_planes
(kms_cursor_crc:2317) igt_kms-DEBUG: Test requirement passed: plane_idx >= 0 && plane_idx < pipe->n_planes
(kms_cursor_crc:2317) igt_kms-DEBUG: display: A.0: plane_set_fb(37)
(kms_cursor_crc:2317) igt_kms-DEBUG: display: A.0: plane_set_size (1024x768)
(kms_cursor_crc:2317) igt_kms-DEBUG: display: A.0: fb_set_position(0,0)
(kms_cursor_crc:2317) igt_kms-DEBUG: display: A.0: fb_set_size(1024x768)
(kms_cursor_crc:2317) igt_kms-DEBUG: display: commit {
(kms_cursor_crc:2317) igt_kms-DEBUG: display:     Virtual-1: SetCrtc pipe A, fb 37, src (0, 0), mode 1024x768
(kms_cursor_crc:2317) igt_kms-DEBUG: display:     SetCursor pipe A, disabling
(kms_cursor_crc:2317) igt_kms-DEBUG: display:     MoveCursor pipe A, (0, 0)
(kms_cursor_crc:2317) igt_kms-DEBUG: display: }
(kms_cursor_crc:2317) igt_debugfs-DEBUG: Opening debugfs directory '/sys/kernel/debug/dri/0'
(kms_cursor_crc:2317) igt_debugfs-DEBUG: Opening debugfs directory '/sys/kernel/debug/dri/0'
(kms_cursor_crc:2317) igt_debugfs-DEBUG: Opening debugfs directory '/sys/kernel/debug/dri/0'
(kms_cursor_crc:2317) igt_core-INFO: Timed out: Opening crc fd, and poll for first CRC.
****  END  ****
Subtest pipe-A-cursor-alpha-opaque: FAIL (10.017s)

---

What I found was a timeout in the poll of
igt_debugfs/igt_pipe_crc_start() that in turn is called in the
prepare_crc() of kms_cursor_crc.

Initially, I thought it was just a matter of timing for vblank and sent
a patch to IGT that solved the problem by calling igt_wait_for_vblank()
before the "start" op. But then I saw that the timeout also happens in a
similar way in the dpms subtest, which before the change to flip_done
was succeded.

I still have doubts if it is or not a matter of timing...

Thanks,

Melissa
> 
> Also happy to chat on irc for debugging ideas, that might be faster (I'm
> danvet on #dri-devel on freenode).
> -Daniel
> 
> > 
> > Melissa
> > 
> > > -Daniel
> > > 
> > > > +
> > > >  	drm_atomic_helper_cleanup_planes(dev, old_state);
> > > >  }
> > > >  
> > > > -- 
> > > > 2.17.1
> > > > 
> > > 
> > > -- 
> > > Daniel Vetter
> > > Software Engineer, Intel Corporation
> > > http://blog.ffwll.ch
> > > _______________________________________________
> > > dri-devel mailing list
> > > dri-devel@lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch