Hi Sylwester,
Sylwester Nawrocki wrote:
On 09/16/2012 05:33 PM, Laurent Pinchart wrote:
On Sunday 16 September 2012 15:57:14 Hans Verkuil wrote:
On Sat September 15 2012 22:16:24 Sylwester Nawrocki wrote:
On 09/15/2012 02:35 PM, Hans Verkuil wrote:
If we switch all existing drivers to monotonic timestamps in kernel
release
3.x, v4l2-compliance can just use the version it gets from
VIDIOC_QUERYCAP and enforce monotonic timestamps verification if the
version is>= 3.x. This isn't more difficult for apps to check than a
dedicated flag (although it's less explicit).
I think that checking for the driver (kernel) version is a very poor
substitute for testing against a proper flag.
That flag should be the default in this case. The flag should be set by
the framework instead giving every driver the job of setting it.
One alternative might be to use a v4l2_buffer flag instead. That does
have the advantage that in the future we can add additional flags
should we need to support different clocks. Should we ever add
support to switch clocks dynamically, then a buffer flag is more
suitable than a driver capability. In that scenario it does make real
sense to have a flag (or really mask).
Say something like this:
/* Clock Mask */
V4L2_BUF_FLAG_CLOCK_MASK 0xf000
/* Possible Clocks */
V4L2_BUF_FLAG_CLOCK_SYSTEM 0x0000
I realized that this should be called:
V4L2_BUF_FLAG_CLOCK_UNKNOWN 0x0000
With a comment saying that is clock is either the system clock or a
monotonic clock. That reflects the current situation correctly.
V4L2_BUF_FLAG_CLOCK_MONOTONIC 0x1000
There is already lots of overhead related to the buffers management, could
we perhaps have the most common option defined in a way that drivers don't
need to update each buffer's flags before dequeuing, only to indicate the
timestamp type (other than flags being modified in videobuf) ?
Well, if all vb2 drivers use the monotonic clock, then you could do it in
__fill_v4l2_buffer: instead of clearing just the state flags you'd clear
state + clock flags, and you OR in the monotonic flag in the case statement
below (adding just a single b->flags |= line in the DEQUEUED case).
So that wouldn't add any overhead. Not that I think setting a flag will add
any measurable overhead in any case.
Yes, that might be indeed negligible overhead, especially if it's done well.
User space logic usually adds much more to complexity.
Might be good idea to add some helpers to videobuf2, so handling timestamps
is as simple as possible in drivers.
Of the V4L2 core. Taking the timestamp has to be done usually at a very
precise point in the code, and that's a decision I think is better done
in the driver. Timestamps are also independent of the videobuf2.
This buffer flags idea sounds to me worse than the capability flag. After
all the drivers should use monotonic clock timestamps, shouldn't they ?
Yes. But you have monotonic and raw monotonic clocks at the moment, and
perhaps others will be added in the future. You can't change clocks if you
put this in the querycap capabilities.
Fair enough. BTW, CLOCK_MONOTONIC_RAW is not defined in any POSIX standard,
is it ?
It's Linux-specific. Perhaps it's worth noting that both V4L2 and ALSA
are Linux-specific, too. :-)
Raw wonotonic time could be better in some use cases as it's not
NTP-adjusted. Which one is better for the purpose might be
system-specific, albeit I'm leaning on the side of the monotonic in a
general case.
...
I'd really like to keep this door open. My experience is that if something
is possible, then someone somewhere will want to use it.
Indeed, caps flags approach might be too limited anyway. And a v4l2 control
might be not good for reporting things like these.
Why not? Are there other mechanisms that are suitable for this than
controls? If we end up using controls for this, then we should make it
as easy as possible for the drivers.
Kind regards,
--
Sakari Ailus
sakari.ailus@xxxxxx
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