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

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Date:	Thu, 9 Oct 2014 12:08:12 +0200
From:	Thierry Reding <treding@...dia.com>
To:	Alexandre Courbot <acourbot@...dia.com>
CC:	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>,
	Russell King <rmk+kernel@....linux.org.uk>,
	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 02:43:02PM +0900, Alexandre Courbot wrote:
> On 10/02/2014 08:52 PM, Andrzej Hajda wrote:
> >Hi,
> >
> >+CC possible victims
> >
> >On 10/02/2014 12:52 PM, Inki Dae wrote:
> >>On 2014년 10월 02일 17:58, Joonyoung Shim wrote:
> >>>Hi Andrzej,
> >>>
> >>>On 10/01/2014 05:14 PM, Andrzej Hajda wrote:
> >>>>The patch disables vblanks during dpms off only if pagefilp has
> >>>>not been finished. It also replaces drm_vblank_off with drm_crtc_vblank_put.
> >>>>It fixes issue with page_flip ioctl not being able to acquire vblank counter.
> >>>This problem isn't related with pageflip, it just causes from
> >>>7ffd7a68511c710b84db3548a1997fd2625f580a commit (drm: Always reject
> >>>drm_vblank_get() after drm_vblank_off()).
> >>>
> >>>We need to use drm_vblank_on() as a counterpart to drm_vblank_off()
> >>>after the commit .
> >
> >This patch should break also other drivers, it seems at least following
> >drms could be affected:
> >armada, sti, tegra.
> 
> Indeed we (tegra) have just been hit by this. The problem seems to come from
> the fact that we have been using drm_vblank_pre_modeset,
> drm_vblank_post_modeset and drm_vblank_off conjointly. All these functions
> depend on the value of vblank->inmodeset, and 7ffd7a68511 increases the
> vblank reference counter only in drm_vblank_off, which can result in the
> acquired reference never being released.
> 
> The following seems to fix this for Tegra, by stopping using
> drm_vblank_pre/post_modeset and relying on drm_vblank_off/on instead:
> 
> diff --git a/drivers/gpu/drm/tegra/dc.c b/drivers/gpu/drm/tegra/dc.c
> index b08df07cad47..3955d81236d0 100644
> --- a/drivers/gpu/drm/tegra/dc.c
> +++ b/drivers/gpu/drm/tegra/dc.c
> @@ -739,7 +739,6 @@ static const struct drm_crtc_funcs tegra_crtc_funcs = {
> 
>  static void tegra_crtc_disable(struct drm_crtc *crtc)
>  {
> -       struct tegra_dc *dc = to_tegra_dc(crtc);
>         struct drm_device *drm = crtc->dev;
>         struct drm_plane *plane;
> 
> @@ -755,7 +754,7 @@ static void tegra_crtc_disable(struct drm_crtc *crtc)
>                 }
>         }
> 
> -       drm_vblank_off(drm, dc->pipe);
> +       drm_crtc_vblank_off(crtc);
>  }
> 
>  static bool tegra_crtc_mode_fixup(struct drm_crtc *crtc,
> @@ -844,7 +843,7 @@ static int tegra_crtc_mode_set(struct drm_crtc *crtc,
>         u32 value;
>         int err;
> 
> -       drm_vblank_pre_modeset(crtc->dev, dc->pipe);
> +       drm_crtc_vblank_off(crtc);
> 
>         err = tegra_crtc_setup_clk(crtc, mode);
>         if (err) {
> @@ -946,7 +945,7 @@ static void tegra_crtc_commit(struct drm_crtc *crtc)
>         value = GENERAL_ACT_REQ | WIN_A_ACT_REQ;
>         tegra_dc_writel(dc, value, DC_CMD_STATE_CONTROL);
> 
> -       drm_vblank_post_modeset(crtc->dev, dc->pipe);
> +       drm_crtc_vblank_on(crtc);
>  }
> 
>  static void tegra_crtc_load_lut(struct drm_crtc *crtc)
> 
> Thierry, does this look ok to you?

Yes, that looks like almost the same patch that I sent out yesterday.
The difference is that I didn't replace the drm_vblank_pre_modeset()
call with drm_vblank_off() like you did, but rather just dropped the
former.

I /think/ your version is more correct in that regard.

Thierry

> 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?

It shouldn't happen. If drm_vblank_off() and drm_vblank_on() are called
around a modeset they should never conflict with drm_vblank_get(), at
least on Tegra, because the modeset and page-flip IOCTLs will be
serialized.

Thierry

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