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

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Date:   Tue, 12 Jul 2022 09:40:41 +0200
From:   Christian König <christian.koenig@....com>
To:     John Stultz <jstultz@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Jason Ekstrand <jason@...kstrand.net>,
        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: Re: [RFC][PATCH 1/3] drm: drm_syncobj: Add note in DOC about absolute
 timeout values

Am 12.07.22 um 06:22 schrieb John Stultz:
> 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.

Well absolute timeout values are mandatory for making -ERESTARTSYS work 
without any additional handling.

So using them is recommended for ~20 years now and IIRC even documented 
somewhere.

See here as well https://lwn.net/Articles/17744/ how much trouble system 
calls with relative timeouts are.

Regards,
Christian.

>
> 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
>    * -------------------------