linux-kernel - Re: [PATCH v3] drm/vkms: guarantee vblank when capturing crc

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Message-ID: <20200810125102.GO2352366@phenom.ffwll.local>
Date:   Mon, 10 Aug 2020 14:51:02 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Melissa Wen <melissa.srw@...il.com>
Cc:     Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>,
        Haneen Mohammed <hamohammed.sa@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Rodrigo Siqueira <Rodrigo.Siqueira@....com>,
        Sidong Yang <realwakka@...il.com>, twoerner@...il.com,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        kernel-usp@...glegroups.com
Subject: Re: [PATCH v3] drm/vkms: guarantee vblank when capturing crc

On Sat, Aug 08, 2020 at 09:09:00AM -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 subtests: 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.
> 
> Cc: Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
> Cc: Haneen Mohammed <hamohammed.sa@...il.com>
> 
> Co-debugged-by: Sidong Yang <realwakka@...il.com>
> Signed-off-by: Sidong Yang <realwakka@...il.com>
> Signed-off-by: Melissa Wen <melissa.srw@...il.com>
> Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>
> 
> ---
> 
> v2:
> - extract a vkms_set_composer helper
> - fix vblank refcounting for the disabling case
> 
> v3:
> - make the vkms_set_composer helper static
> - review the credit tags

Thanks a lot for your work here, patch merged to drm-misc-next.

I've kept the changelog in the commit message, I often find that quite
useful. But that's a bit a drm peculiarity, most other subsystems prefer
it like you've done it.

Cheers, Daniel

> 
> ---
>  drivers/gpu/drm/vkms/vkms_composer.c | 20 +++++++++++++++++---
>  1 file changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> index b8b060354667..4f3b07a32b60 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;
>  }
>  
> +static void vkms_set_composer(struct vkms_output *out, bool enabled)
> +{
> +	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;
>  }
> -- 
> 2.27.0
> 

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