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

From: Alexandre Courbot
Date: Thu Oct 09 2014 - 01:43:53 EST

On 10/02/2014 08:52 PM, Andrzej Hajda wrote:

+CC possible victims

On 10/02/2014 12:52 PM, Inki Dae wrote:
On 2014ë 10ì 02ì 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)
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?

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?
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