Re: [PATCH 14/19] drm: Don't update vblank timestamp when the counter didn't change

[Mario Kleiner <mario.kleiner.de@gmail.com>]

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The current drm-next misses Ville's original Patch 14/19, the one i 
first objected, then objected to my objection. It is needed to avoid 
actual regressions. Attached a trivially rebased (v2) of Ville's patch 
to go on top of drm-next, also as tgz in case my e-mail client mangles 
the patch again, because it's one of those "email hates me" weeks.

-mario



On 08/06/2014 01:49 PM, ville.syrjala@linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>
> If we already have a timestamp for the current vblank counter, don't
> update it with a new timestmap. Small errors can creep in between two
> timestamp queries for the same vblank count, which could be confusing to
> userspace when it queries the timestamp for the same vblank sequence
> number twice.
>
> This problem gets exposed when the vblank disable timer is not used
> (or is set to expire quickly) and thus we can get multiple vblank
> disable<->enable transition during the same frame which would all
> attempt to update the timestamp with the latest estimate.
>
> Testcase: igt/kms_flip/flip-vs-expired-vblank
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>   drivers/gpu/drm/drm_irq.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index af33df1..0523f5b 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -106,6 +106,9 @@ static void drm_update_vblank_count(struct drm_device *dev, int crtc)
>   	DRM_DEBUG("enabling vblank interrupts on crtc %d, missed %d\n",
>   		  crtc, diff);
>   
> +	if (diff == 0)
> +		return;
> +
>   	/* Reinitialize corresponding vblank timestamp if high-precision query
>   	 * available. Skip this step if query unsupported or failed. Will
>   	 * reinitialize delayed at next vblank interrupt in that case.


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>From c0a5228a7fc43d4c3615a471c340b68bcb2caa16 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ville=20Syrj=C3=A4l=C3=A4?= <ville.syrjala@linux.intel.com>
Date: Wed, 6 Aug 2014 14:49:57 +0300
Subject: [PATCH] drm: Don't update vblank timestamp when the counter didn't
 change (v2)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

If we already have a timestamp for the current vblank counter, don't
update it with a new timestmap. Small errors can creep in between two
timestamp queries for the same vblank count, which could be confusing to
userspace when it queries the timestamp for the same vblank sequence
number twice.

This problem gets exposed when the vblank disable timer is not used
(or is set to expire quickly) and thus we can get multiple vblank
disable<->enable transition during the same frame which would all
attempt to update the timestamp with the latest estimate.

Testcase: igt/kms_flip/flip-vs-expired-vblank
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>

v2:Mario: Trivial rebase on top of current drm-next (13-Sep-2014)
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
---
 drivers/gpu/drm/drm_irq.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 80ff94a..e73cbda 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -126,6 +126,9 @@ static void drm_update_vblank_count(struct drm_device *dev, int crtc)
 	DRM_DEBUG("updating vblank count on crtc %d, missed %d\n",
 		  crtc, diff);
 
+	if (diff == 0)
+		return;
+
 	/* Reinitialize corresponding vblank timestamp if high-precision query
 	 * available. Skip this step if query unsupported or failed. Will
 	 * reinitialize delayed at next vblank interrupt in that case.
-- 
1.9.1


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