Re: [PATCH] drm/vkms: guarantee vblank when capturing crc

[Melissa Wen <melissa.srw@gmail.com>]

On 08/04, daniel@ffwll.ch wrote:
> On Sat, Aug 01, 2020 at 03:49:29PM -0300, Melissa Wen wrote:
> > VKMS needs vblank interrupts enabled to capture CRC. When vblank is
> > disabled, tests like kms_cursor_crc and kms_pipe_crc_basic getting stuck
> > waiting for a capture that will not occur until vkms wakes up. This
> > patch ensures that vblank remains enabled as long as the CRC capture is
> > needed.
> > 
> > It clears the execution of the following kms_cursor_crc subtests:
> > 1. pipe-A-cursor-[size,alpha-opaque, NxN-(on-screen, off-screen, sliding,
> > random, fast-moving])] - successful when running individually.
> > 2. pipe-A-cursor-dpms passes again
> > 3. pipe-A-cursor-suspend also passes
> > 
> > The issue was initially tracked in the sequential execution of IGT
> > kms_cursor_crc subtest: when running the test sequence or one of its
> > subtests twice, the odd execs complete and the pairs get stuck in an
> > endless wait. In the IGT code, calling a wait_for_vblank on preparing
> > for CRC capture prevented the busy-wait. But the problem persisted in
> > the pipe-A-cursor-dpms and -suspend subtests.
> > 
> > Checking the history, the pipe-A-cursor-dpms subtest was successful
> > when, in vkms_atomic_commit_tail, instead of using the flip_done op, it
> > used wait_for_vblanks. Another way to prevent blocking was
> > wait_one_vblank when enabling crtc. However, in both cases,
> > pipe-A-cursor-suspend persisted blocking in the 2nd start of CRC
> > capture, which may indicate that something got stuck in the step of CRC
> > setup. Indeed, wait_one_vblank in the crc setup was able to sync things
> > and free all kms_cursor_crc subtests.
> > 
> > Besides, other alternatives to force enabling vblanks or prevent
> > disabling them such as calling drm_crtc_put_vblank or modeset_enables
> > before commit_planes + offdelay = 0, also unlock all subtests
> > executions. These facts together converge on the lack of vblank to
> > unblock the crc capture.
> > 
> > Finally, considering the vkms's dependence on vblank interruptions to
> > perform tasks, this patch acquires a vblank ref when setup CRC source
> > and releases ref when disabling crc capture, ensuring vblanks happen to
> > compute CRC.
> > 
> > Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
> > Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
> > Co-developed-by: Sidong Yang <realwakka@gmail.com>
> > Signed-off-by: Sidong Yang <realwakka@gmail.com>
> > Co-developed-by: Daniel Vetter <daniel@ffwll.ch>
> > Signed-off-by: Daniel Vetter <daniel@ffwll.ch>
> > Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
> 
> Excellent summary of the debug story.
> 
> > ---
> >  drivers/gpu/drm/vkms/vkms_composer.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> > index 4af2f19480f4..1161eaa383f1 100644
> > --- a/drivers/gpu/drm/vkms/vkms_composer.c
> > +++ b/drivers/gpu/drm/vkms/vkms_composer.c
> > @@ -241,6 +241,14 @@ int vkms_set_crc_source(struct drm_crtc *crtc, const char *src_name)
> >  
> >  	ret = vkms_crc_parse_source(src_name, &enabled);
> >  
> > +	/* Ensure that vblank interruptions are enabled for crc capture */
> > +	/* Enabling CRC: acquire vblank ref */
> 
> This comment just explains what the code does, that's not needed. The
> first comment I think can be replaced if we create a little helper
> function like
> 
> void vkms_set_composer(struct vkms_output, bool enable) {
> 	bool old_state;
> 
> 	if (enabled)
> 		drm_crtc_vblank_get(crtc);
> 
> 	spin_lock_irq(&out->lock);
> 	old_enable = out->composer_enabled;
> 	out->composer_enabled = enabled;
> 	spin_unlock_irq(&out->lock);
> 
> 	if (old_enabled)
> 		drm_crtc_vblank_put(crtc);
> 
> }
> 
> This should also help as prep for the writeback work, where we have a 2nd
> user that might need to enable the composer (maybe even need to switch to
> refcounting the composer state then).
>
Hi Daniel,

Thanks for the feedback and advice.
I applied the suggestion and just sent a v2.

Best regards,

Melissa

> > +	if (enabled)
> > +		drm_crtc_vblank_get(crtc);
> > +	/* Disabling CRC: release vblank ref */
> > +	if (!src_name)
> > +		drm_crtc_vblank_put(crtc);
> 
> I'm not sure this correctly releases the vblank reference in all cases, I
> think the suggestion in the helper function pseudo code should work
> better. It does mean we temporarily elevate the vblank refcount if we go
> enabled -> enabled, but that's not a problem since it's refcounted.
> 
> Cheers, Daniel
> 
> > +
> >  	spin_lock_irq(&out->lock);
> >  	out->composer_enabled = enabled;
> >  	spin_unlock_irq(&out->lock);
> > -- 
> > 2.27.0
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch