linux-kernel - [RFC][PATCH 1/3] drm: drm_syncobj: Add note in DOC about absolute timeout values

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Date:   Tue, 12 Jul 2022 04:22:56 +0000
From:   John Stultz <jstultz@...gle.com>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     John Stultz <jstultz@...gle.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Jason Ekstrand <jason@...kstrand.net>,
        "Christian König" <christian.koenig@....com>,
        Lionel Landwerlin <lionel.g.landwerlin@...el.com>,
        Chunming Zhou <david1.zhou@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org
Subject: [RFC][PATCH 1/3] drm: drm_syncobj: Add note in DOC about absolute
 timeout values

After having to debug down through the kernel to figure out
why my _WAIT calls were always timing out, I realized its
an absolute timeout value instead of the more common relative
timeouts.

This detail should be called out in the documentation, as while
the absolute value makes sense here, its not as common for timeout
values.

Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
Cc: Maxime Ripard <mripard@...nel.org>
Cc: Thomas Zimmermann <tzimmermann@...e.de>
Cc: Jason Ekstrand <jason@...kstrand.net>
Cc: Christian König <christian.koenig@....com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@...el.com>
Cc: Chunming Zhou <david1.zhou@....com>
Cc: David Airlie <airlied@...ux.ie>
Cc: Daniel Vetter <daniel@...ll.ch>
Cc: dri-devel@...ts.freedesktop.org
Signed-off-by: John Stultz <jstultz@...gle.com>
---
 drivers/gpu/drm/drm_syncobj.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index 7e48dcd1bee4..b84d842a1c21 100644
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -136,6 +136,10 @@
  * requirement is inherited from the wait-before-signal behavior required by
  * the Vulkan timeline semaphore API.
  *
+ * It should be noted, that both &DRM_IOCTL_SYNCOBJ_WAIT and
+ * &DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT takes an *absolute* CLOCK_MONOTONIC
+ * nanosecond value for the timeout value. Accidentally passing relative time
+ * values will likely result in an immediate -ETIME return.
  *
  * Import/export of syncobjs
  * -------------------------
-- 
2.37.0.144.g8ac04bfd2-goog