can VCHI frame timestamp values be made opaque? (not usec)
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Assessment
- Difficulty
- 5/5
- Estimated time
- Over a week
- Newbie friendliness
- 25/100
- Issue type
- Feature
- Clarity
- Needs clarification
- Activity status
- Stale
- Tech stack
- linux
- Domain
- embedded-iot, operating-systems
Research direction
Start by tracing the bcm2835-codec timestamp path through MMAL/VCHI and reviewing the linked linux-media discussion. Determine whether firmware interprets the timestamp as microseconds; done means either establishing that it can remain opaque or identifying the required separate field if microsecond values are needed.
Written by the indexing model from the issue text.
Description
While upstreaming bcm2835-codec, Nicolas Dufresne noted that the timestamp in v4l2 is not necessarily an actual time value (in nsec). In fact, GStreamer actually uses a frame counter for this field. (see https://lore.kernel.org/linux-media/20240303152635.2762696-1-maarten@rmail.be/T/#m4521f17f9e75074ff3993df4a23c146c26f6e2a6 )
I don't know how the firmware handle the mmal(VCHI?) timestamp values, does it actually calculate something with these timestamps?
If it doesn't, it might be easy enough to get a fix in the firmware to not "interprete" these timestamp values, which would make the mmal code a lot simpler and could avoid potential issues (eg: GStreamer)
The code does state in comment that the mmal timestamps are usec, so if it does need usec timestamps and it needs this for a specific calculation, maybe we can add an extra field?
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