Hi Sylwester,
Sylwester Nawrocki wrote:
On 09/17/2012 07:19 PM, Sakari Ailus wrote:
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:
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.
Yes, good point. All in all videobuf2 belongs to v4l2-core, doesn't
it ? ;)
You're correct. I meant to say that it could (or should) be separate
from handling the buffers themselves.
Taking a timestamp indeed needs some care and precision, but setting
a flag could be considered a sort of separate issue - it's more relaxed
and videobuf2 already handles the buffer flags.
True.
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. :-)
OK. I don't mind V4L2 and ALSA being Linux-specific...
:)
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.
Yeah, I guess it's all determined by streams from what subsystems
we're trying to synchronize and what clocks are used there. If there
is a possibility to select from various clocks in at least one of
the subsystems then we're all set.
It's not only that, it's also that the clock has to be suitable for the
synchronisation problem at hand. Currently realtime timestamps could be
used by ALSA and V4L2 but I could hardly recommend using them for
audio/video synchronisation.
The main issue here is that we already have plenty of different
clocks and there is a need on the video side for at least:
1. reporting to user space what clock is used by a driver,
and optionally
2. selecting clock type on user request.
I think the solution for 1 should be such it makes easy and clean to do 2.
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.
Sorry, my concern here was that timestamps are needed by all video
devices and I wasn't sure if there are any video nodes that don't
implement the v4l2 control ioctls. I.e. we might be enforcing adding
controls support only for the purpose of being able to query the
timestamps type. That was my concern here about using controls. However
if all video devices implement the controls API then it's negligible.
Moreover some parts of such control implementation could likely be
a part v4l2-core.
I'm just wondering why we need a flag when a control is going to be
used anyway. It sounds like per-buffer controls/status but that's an
issue that was previously discussed and is still not really addressed
in V4L2 AFAICT.
Flags + a control is likely going to fulfil all (most of) possible
app requirements. Not sure if the applications really need to get
timestamp type from each v4l2 buffer and the drivers need to be setting
it. Rather than just using a control before starting/after stopping
streaming to select, and at any time to query, the clock type.
I think so, too. It's unlikely that the user would want to change the
value of the timestamp type control while streaming. Those flags could
well be added later on if the need to do so arises.
Kind regards,
--
Sakari Ailus
sakari.ailus@xxxxxx
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