[RFC 1/4] time: export getnstime_raw_and_real for DRM

Mario Kleiner mario.kleiner at tuebingen.mpg.de
Fri Oct 5 18:41:57 PDT 2012

> Subject: Re: [RFC 4/4] drm: add support for raw monotonic vblank
> 	timestamps
> Message-ID: <1349446447.17758.73.camel at thor.local>
> Content-Type: text/plain; charset="ISO-8859-1"
> On Fre, 2012-10-05 at 16:59 +0300, Imre Deak wrote:
>> On Fri, 2012-10-05 at 15:55 +0200, Michel D?nzer wrote:
>>> On Fre, 2012-10-05 at 16:37 +0300, Imre Deak wrote:
>>>> In practice we never want the timestamps for vblank and page flip events
>>>> to be affected by time adjustments, so in addition to the gettimeofday
>>>> timestamps we used so far add support for raw monotonic timestamps.
>>>> For backward compatibility use flags to select between the old and new
>>>> timestamp format.
>>>> Note that with this change we will save the timestamp in both formats,
>>>> for cases where multiple clients are expecting an event notification in
>>>> different time formats.
>>> I wonder if all this trouble is really necessary. I honestly can't
>>> imagine any user of this API requiring non-monotonic timestamps and
>>> breaking with monotonic ones. I think it was simply a mistake that we
>>> didn't make them monotonic in the first place (or maybe it wasn't even
>>> possible when this API was first introduced).


The OML_sync_control spec requires monotonic timestamps, so yes, it was 
either a mistake or it wasn't possible at that time.

If an app developer follows the spec, he would consider the current 
behaviour broken -- or should be able to handle monotonic timestamps.

>> Yea, I'd rather simply switch over to monotonic timestamps too. But that
>> would break apps that already compare against the wall time for whatever
>> purpose (for example A/V sync).
> Are there actually any such apps in the real world? Do they work when
> the wall time jumps?

Psychtoolbox as my example of a very timing sensitive app: Currently 
uses wall time as a reference for historical reasons. We tell/expect 
users not to do anything in the middle of a work session that could 
cause a sudden jump in wall time, but then ntp time adjustments are 
usually in very small steps, so not a problem, and big adjustments 
should only happen if users do something they'll certainly not do during 
a timing-sensitve session, like manually set the system clock, 
reboot/suspend/resume the machine etc. In practice, this works well 
enough over the typical duration of such sessions (minutes to a few 
hours) and is sometimes needed if multiple machines need to be 
time-synced over the local network.

But then Psychtoolbox checks each timestamp it gets from somewhere 
"outside" (OML_sync_control / INTEL_swap_events / ALSA audio timestamps, 
network receive timestamps, evdev, x11, ...) if it is in gettimeofday() 
aka CLOCK_REALTIME aka wall time or in CLOCK_MONOTONIC time and just 
remaps to whatever its reference clock is.

There's no way around this than to have no fixed expectations, but to 
remap stuff on the fly, because different parts of the Linux universe 
have decided on different time bases, or even switched somewhere from 
one kernel version to the next in the last years, e.g., ALSA, which at 
some time switched from wall clock to CLOCK_MONOTONIC. Sometimes 
clock_gettime() wasn't available at all in older setups, so there only 
was gettimeofday(). Or toolkits like GStreamer use different timebases 
dependent on OS and sometimes even on plugins.

I would expect that other timing sensitive apps have to have ways to 
handle this in similar ways.

Wrt. to the drm vblank/pageflip timestamps, the userspace extensions 
which expose these (INTEL_swap_events, OML_sync_control) don't allow 
apps to select which timebase to use, they define monotonic time as what 
is returned, so i don't know how a userspace app could actually ask the 
DRM for one or the other format? So i guess just switching to 
CLOCK_MONOTONIC shouldn't be that bad.

Kristian, i assume Wayland will also return presentation timestamps in 
the format and microsecond precision of the DRM, right?

On 05.10.12 18:22, intel-gfx-request at lists.freedesktop.org wrote:
> Message: 7 Date: Fri, 5 Oct 2012 12:14:29 -0400 From: Kristian H?gsberg

> I just had a quick look at driver/input/evdev.c, since evdev devices
> did a similar change recently to allow evdev timestamp from the
> monotonic clock.  They're using a different time API:
> 	time_mono = ktime_get();
>          time_real = ktime_sub(time_mono, ktime_get_monotonic_offset());
> and
>          event->time = ktime_to_timeval(client->clkid == CLOCK_MONOTONIC ?
> 			                mono : real);
> I'm not really up-to-date on kernel time APIs, but I wonder if that
> may be better?  At least, I suspect we wouldn't need changes outside
> drm if we use this API.
> Kristian

Userspace apps only have access to what gettimeofday() and 
clock_gettime() for CLOCK_REALTIME (== gettimeofday() afaik) and 
CLOCK_MONOTONIC return, so whatever is returned should be in 
CLOCK_MONOTONIC format, otherwise there will be lots of tears and dead 
kittens. I think what evdev does makes a lot of sense, but i'm also not 
up-to-date about the various layers of timing apis.


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