On Mon, Nov 27, 2023 at 4:12 PM Richard Cochran
[off-list ref] wrote:

On Mon, Nov 27, 2023 at 05:39:01PM +0200, Sagi Maimon wrote:

quoted

 Some user space applications need to read some clocks.
 Each read requires moving from user space to kernel space.
 This asymmetry causes the measured offset to have a significant error.

Adding time/clock gurus (jstultz, tglx) on CC for visibility...

Thanks for the heads up! (though, "guru" is just the noise I make
standing up these days)

quoted

 Introduce a new system call multi_clock_gettime, which can be used to measure
 the offset between multiple clocks, from variety of types: PHC, virtual PHC
 and various system clocks (CLOCK_REALTIME, CLOCK_MONOTONIC, etc).
 The offset includes the total time that the driver needs to read the clock
 timestamp.

This last bit about "offset includes the total time that the driver
needs to read the clock" is a bit confusing. It seems to suggest there
would be start/stop bookend timestamps so you could bound how long it
took to read all the clocks, but I don't see that in the patch.

quoted

 New system call allows the reading of a list of clocks - up to PTP_MAX_CLOCKS.
 Supported clocks IDs: PHC, virtual PHC and various system clocks.
 Up to PTP_MAX_SAMPLES times (per clock) in a single system call read.
 The system call returns n_clocks timestamps for each measurement:
 - clock 0 timestamp
 - ...
 - clock n timestamp

Signed-off-by: Sagi Maimon <redacted>

Overally, while I understand the intent, I'm pretty hesitant on it
(and "__ptp_multi_clock_get multi_clk_get" has me squinting to find
the actual space amongst all the underscores :).

If the overhead of reading clockids individually is too much, it seems
like the next thing will be folks wanting to export multiple raw
hardware counter values so the counter->ns transformation doesn't get
inbetween each hw clock read, which this interface wouldn't solve, so
we'd have to add yet another interface.

Also, I wonder if trying to get multiple clocks in one read seems
similar to something uio_ring might help with? Though I can't say I'm
very savvy with uio_ring. Have folks looked into that?

thanks
-john