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It takes at least 2.2us to get this value back to > userspace. At the same time there is cached value of upper bits of PHC > available for packet timestamps already. This patch reuses cached value > to speed up reading of PHC time. > > Signed-off-by: Vadim Fedorenko > --- > v1 -> v2: > * move cycles extension to a helper function and reuse it for both > timestamp extension and gettimex64() function > > I did some benchmarks on host with Broadcom Thor NIC trying to build > histogram of time spent to call clock_gettime() to query PTP device > over million iterations. > With current implementation the result is (long tail is cut): > > 2200ns: 902624 > 2300ns: 87404 > 2400ns: 4025 > 2500ns: 1307 > 2600ns: 581 > 2700ns: 261 > 2800ns: 104 > 2900ns: 36 > 3000ns: 32 > 3100ns: 24 > 3200ns: 16 > 3300ns: 29 > 3400ns: 29 > 3500ns: 23 > > Optimized version on the very same machine and NIC gives next values: > > 900ns: 865436 > 1000ns: 128630 > 1100ns: 2671 > 1200ns: 727 > 1300ns: 397 > 1400ns: 178 > 1500ns: 92 > 1600ns: 16 > 1700ns: 15 > 1800ns: 11 > 1900ns: 6 > 2000ns: 20 > 2100ns: 11 > > That means pct(99) improved from 2300ns to 1000ns. > --- The driver already has to read and cache the values, so there's not much value in repeating that every CLOCK_GETTIME system call. This also simplifies the system timestamp process, and avoids the duplicate reads. Clever! Reviewed-by: Jacob Keller