Re: [PATCH v2] drm/vkms: guarantee vblank when capturing crc

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Aug 06, 2020 at 08:17:05AM -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
> adds a helper to set composer and ensure 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.
> 
> Finally, due to vkms's dependence on vblank interruptions to perform
> tasks, this patch uses refcount to ensure that vblanks happen when
> enabling composer and while crc capture is needed.

For next time around, please include a patch changelog here so people know
what was changed, and why. E.g.

v2:
- extract a vkms_set_composer helper
- fix vblank refcounting for the disabling case

> 
> 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>

Nah that's a bit too much credit for me, I'll get the r-b tag. You &
Sidong figured this story out, so I'd use Co-debugged-by: Sidong here for
credits.

> Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
> ---
>  drivers/gpu/drm/vkms/vkms_composer.c | 20 +++++++++++++++++---
>  drivers/gpu/drm/vkms/vkms_drv.h      |  1 +
>  2 files changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> index b8b060354667..e2ac2b9759bf 100644
> --- a/drivers/gpu/drm/vkms/vkms_composer.c
> +++ b/drivers/gpu/drm/vkms/vkms_composer.c
> @@ -233,6 +233,22 @@ int vkms_verify_crc_source(struct drm_crtc *crtc, const char *src_name,
>  	return 0;
>  }
>  
> +void vkms_set_composer(struct vkms_output *out, bool enabled)

Since this isn't yet used outside of this file I'd just make this static.
Writeback connector support can then add the prototype in the header. With
that

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Please include that tag for revision 3.

Cheers, Daniel

> +{
> +	bool old_enabled;
> +
> +	if (enabled)
> +		drm_crtc_vblank_get(&out->crtc);
> +
> +	spin_lock_irq(&out->lock);
> +	old_enabled = out->composer_enabled;
> +	out->composer_enabled = enabled;
> +	spin_unlock_irq(&out->lock);
> +
> +	if (old_enabled)
> +		drm_crtc_vblank_put(&out->crtc);
> +}
> +
>  int vkms_set_crc_source(struct drm_crtc *crtc, const char *src_name)
>  {
>  	struct vkms_output *out = drm_crtc_to_vkms_output(crtc);
> @@ -241,9 +257,7 @@ int vkms_set_crc_source(struct drm_crtc *crtc, const char *src_name)
>  
>  	ret = vkms_crc_parse_source(src_name, &enabled);
>  
> -	spin_lock_irq(&out->lock);
> -	out->composer_enabled = enabled;
> -	spin_unlock_irq(&out->lock);
> +	vkms_set_composer(out, enabled);
>  
>  	return ret;
>  }
> diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
> index f4036bb0b9a8..2cc86d08bd4e 100644
> --- a/drivers/gpu/drm/vkms/vkms_drv.h
> +++ b/drivers/gpu/drm/vkms/vkms_drv.h
> @@ -142,6 +142,7 @@ int vkms_verify_crc_source(struct drm_crtc *crtc, const char *source_name,
>  			   size_t *values_cnt);
>  
>  /* Composer Support */
> +void vkms_set_composer(struct vkms_output *output, bool enabled);
>  void vkms_composer_worker(struct work_struct *work);
>  
>  #endif /* _VKMS_DRV_H_ */
> -- 
> 2.27.0
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch