[PATCH net-next v3 0/3] ptp: Support for multiple filtered timestamp event queue readers

[Xabier Marquiegui <reibax@gmail.com>]

On systems with multiple timestamp event channels, there can be scenarios where
multiple userspace readers want to access the timestamping data for various
purposes.

One such example is wanting to use a pps out for time synchronization, and
wanting to timestamp external events with the synchronized time base 
simultaneously.

Timestmp event consumers on the other hand, are often interested in a subset of
the available timestamp channels. linuxptp ts2phc, for example, is not happy if
more than one timestamping channel is active on the device it is reading from.

This patch-set introduces linked lists to support multiple timestamp event queue
consumers, and timestamp event channel filters through IOCTLs.

Signed-off-by: Xabier Marquiegui <reibax@gmail.com>
Suggested-by: Richard Cochran <richardcochran@gmail.com>
---
v3:
  - add this patchset overview file
  - fix use of safe and non safe linked lists for loops
  - introduce new posix_clock private_data and ida object ids for better
    dicrimination of timestamp consumers
  - safer resource release procedures
  - filter application by object id, aided by process id
  - friendlier testptp implementation of event queue channel filters
v2: https://lore.kernel.org/netdev/20230912220217.2008895-1-reibax@gmail.com/
  - fix ptp_poll() return value
  - Style changes to comform to checkpatch strict suggestions
  - more coherent ptp_read error exit routines
  - fix testptp compilation error: unknown type name 'pid_t'
  - rename mask variable for easier code traceability
  - more detailed commit message with two examples
v1: https://lore.kernel.org/netdev/20230906104754.1324412-2-reibax@gmail.com/