Merge branch 'ptp-support-hardware-clocks-with-additional-free-running-cycle-counter'

Gerhard Engleder says:

====================
ptp: Support hardware clocks with additional free running cycle counter

ptp vclocks require a clock with free running time for the timecounter.
Currently only a physical clock forced to free running is supported.
If vclocks are used, then the physical clock cannot be synchronized
anymore. The synchronized time is not available in hardware in this
case. As a result, timed transmission with TAPRIO hardware support
is not possible anymore.

If hardware would support a free running time additionally to the
physical clock, then the physical clock does not need to be forced to
free running. Thus, the physical clocks can still be synchronized while
vclocks are in use.

The physical clock could be used to synchronize the time domain of the
TSN network and trigger TAPRIO. In parallel vclocks can be used to
synchronize other time domains.

One year ago I thought for two time domains within a TSN network also
two physical clocks are required. This would lead to new kernel
interfaces for asking for the second clock, ... . But actually for a
time triggered system like TSN there can be only one time domain that
controls the system itself. All other time domains belong to other
layers, but not to the time triggered system itself. So other time
domains can be based on a free running counter if similar mechanisms
like 2 step synchroisation are used.

Synchronisation was tested with two time domains between two directly
connected hosts. Each host run two ptp4l instances, the first used the
physical clock and the second used the virtual clock. I used my FPGA
based network controller as network device. ptp4l was used in
combination with the virtual clock support patches from Miroslav
Lichvar.

v4:
- if_index of 0 is invalid (Jonathan Lemon)
- set if_index to 0 in the SOF_TIMESTAMPING_RAW_HARDWARE block (Jonathan
  Lemon)
- add helper function for netdev_get_tstamp() call (Jonathan Lemon)
- update SKBTX_ANY_TSTAMP (Paolo Abeni)
- use separate bits for new tx_flags (Richard Cochran)

v3:
- optimize ptp_convert_timestamp (Richard Cochran)
- call dev_get_by_napi_id() only if needed (Richard Cochran)
- use non-negated logical test (Richard Cochran)
- add comment for skipped output (Richard Cochran)
- add comment for SKBTX_HW_TSTAMP_USE_CYCLES masking (Richard Cochran)

v2:
- rename ptp_clock cycles to has_cycles (Richard Cochran)
- call it free running cycle counter (Richard Cochran)
- update struct skb_shared_hwtstamps kdoc (Richard Cochran)
- optimize timestamp address/cookie processing path (Richard Cochran,
  Vinicius Costa Gomes)

v1:
- complete rework based on suggestions (Richard Cochran)
====================

Link: https://lore.kernel.org/r/20220506200142.3329-1-gerhard@engleder-embedded.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>