linux-kernel - Re: [PATCH 1/2] drm/atomic: Allow vblank-enabled + self-refresh "disable"

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Date:   Fri, 6 Jan 2023 18:53:49 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Brian Norris <briannorris@...omium.org>
Cc:     Heiko Stübner <heiko@...ech.de>,
        Sean Paul <seanpaul@...omium.org>,
        Michel Dänzer <michel.daenzer@...lbox.org>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Sandy Huang <hjc@...k-chips.com>,
        linux-rockchip@...ts.infradead.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/2] drm/atomic: Allow vblank-enabled + self-refresh
 "disable"

On Thu, Jan 05, 2023 at 05:40:17PM -0800, Brian Norris wrote:
> The self-refresh helper framework overloads "disable" to sometimes mean
> "go into self-refresh mode," and this mode activates automatically
> (e.g., after some period of unchanging display output). In such cases,
> the display pipe is still considered "on", and user-space is not aware
> that we went into self-refresh mode. Thus, users may expect that
> vblank-related features (such as DRM_IOCTL_WAIT_VBLANK) still work
> properly.
> 
> However, we trigger the WARN_ONCE() here if a CRTC driver tries to leave
> vblank enabled here.
> 
> Add a new exception, such that we allow CRTCs to be "disabled" (with
> self-refresh active) with vblank interrupts still enabled.
> 
> Cc: <stable@...r.kernel.org> # dependency for subsequent patch
> Signed-off-by: Brian Norris <briannorris@...omium.org>
> ---
> 
>  drivers/gpu/drm/drm_atomic_helper.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index d579fd8f7cb8..7b5eddadebd5 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -1207,6 +1207,12 @@ disable_outputs(struct drm_device *dev, struct drm_atomic_state *old_state)
>  
>  		if (!drm_dev_has_vblank(dev))
>  			continue;
> +		/*
> +		 * Self-refresh is not a true "disable"; let vblank remain
> +		 * enabled.
> +		 */
> +		if (new_crtc_state->self_refresh_active)
> +			continue;

This very fishy, because we check in crtc_needs_disable whether this
output should stay on due to self-refresh. Which means you should never
end up in here.

And yes vblank better work in self refresh :-) If it doesn't, then you
need to fake it with a timer, that's at least what i915 has done for
transparent self-refresh.

We might need a few more helpers. Also, probably more igt, or is this
something igt testing has uncovered? If so, please cite the igt testcase
which hits this.
-Daniel

>  
>  		ret = drm_crtc_vblank_get(crtc);
>  		WARN_ONCE(ret != -EINVAL, "driver forgot to call drm_crtc_vblank_off()\n");
> -- 
> 2.39.0.314.g84b9a713c41-goog
> 

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