From: Hoegeun Kwon <hoegeun.kwon@samsung.com>
To: daniel.vetter@intel.com, jani.nikula@linux.intel.com,
        seanpaul@chromium.org, airlied@linux.ie
Cc: dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        Hoegeun Kwon <hoegeun.kwon@samsung.com>
Subject: [PATCH] drm/vblank: Fix delta_ns to an absolute value
Date: Fri, 01 Sep 2017 16:07:16 +0900
Message-id: <1504249636-11681-1-git-send-email-hoegeun.kwon@samsung.com>
CMS-TYPE: 101P
DLP-Filter: Pass
References: <CGME20170901070725epcas1p288a5be80c3a3edb02433d24f5beed338@epcas1p2.samsung.com>
Sender: linux-kernel-owner@vger.kernel.org
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Lines: 39

If scanout started, we should reduce etime by delta_ns. But delta_ns
is negative if scanout has not started. If delta_ns is negative,
subtraction of delta_ns from etime increases etime. This is wrong, the
etime should not be increased, so you have to make delta_ns an
absolute value.

Signed-off-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
---

Hello all,

I think that the etime should not be increased.
In cases where delta_ns is negative, if you get time again after an
interrupt call, there is a problem that the time obtained from the
interrupt becomes the future time instead of the past time.

Please let me know if this patch is wrong.

Best regards,
Hoegeun

 drivers/gpu/drm/drm_vblank.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index 70f2b95..a3e0176 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -684,7 +684,7 @@ bool drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev,
 	/* Subtract time delta from raw timestamp to get final
 	 * vblank_time timestamp for end of vblank.
 	 */
-	etime = ktime_sub_ns(etime, delta_ns);
+	etime = ktime_sub_ns(etime, abs(delta_ns));
 	*vblank_time = ktime_to_timeval(etime);
 
 	DRM_DEBUG_VBL("crtc %u : v p(%d,%d)@ %ld.%ld -> %ld.%ld [e %d us, %d rep]\n",
-- 
1.9.1