linux-kernel - Re: [PATCH] drm/exynos: fix vblank handling during dpms off

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Message-ID: <20141009095850.GA3718@ulmo.nvidia.com>
Date:	Thu, 9 Oct 2014 11:58:52 +0200
From:	Thierry Reding <treding@...dia.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Alexandre Courbot <acourbot@...dia.com>,
	Andrzej Hajda <a.hajda@...sung.com>,
	Inki Dae <inki.dae@...sung.com>,
	Joonyoung Shim <jy0922.shim@...sung.com>,
	<linux-samsung-soc@...r.kernel.org>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Seung-Woo Kim <sw0312.kim@...sung.com>,
	<linux-kernel@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Benjamin Gaignard <benjamin.gaignard@...aro.org>,
	Ville Syrjälä <ville.syrjala@...ux.intel.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH] drm/exynos: fix vblank handling during dpms off

On Thu, Oct 09, 2014 at 09:52:58AM +0100, Russell King - ARM Linux wrote:
> On Thu, Oct 09, 2014 at 02:43:02PM +0900, Alexandre Courbot wrote:
> > But there might be another issue, which is that calls to  
> > drm_vblank_get() will return -EINVAL if invoked between drm_blank_off  
> > and drm_blank_on. Is this really the desired behavior? Can it at least  
> > happen? If so, how are drivers supposed to react to this situation?
> 
> I've not yet seen the commit which causes this problem, but I hope
> that drm_wait_vblank() isn't affected by this.  In current mainline,
> drm_vblank_get() is used inside drm_wait_vblank(), which is called as
> a result of userspace calling DRM_IOCTL_WAIT_VBLANK.
> 
> So, what is the effect of this change on user applications making use
> of the vblank wait ioctl - and is that change intended?

There's no effect on user applications if the driver behaves properly.
As far as I can tell, every driver that calls drm_vblank_off() but not
drm_vblank_on() will break. You can easily test this by running libdrm
modetest -s ... -v, which instead of toggling between the test pattern
and an all-gray framebuffer will switch to the gray one once and then
hang.

I guess that was probably not intended, but according to the new rules
all these drivers have now become buggy. So before merging this patch I
think we need to fix existing drivers to avoid regressions.

Thierry

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