Messages in this thread | | | Subject | Re: [PATCH v2] x86/tsc: Convert ART in nanoseconds to TSC. | From | Rajvi Jingar <> | Date | Thu, 8 Mar 2018 17:24:43 -0800 |
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Hi Thomas,
Thank you for your comments.
On Thu, 2018-03-08 at 15:25 +0100, Thomas Gleixner wrote: > On Tue, 6 Mar 2018, Rajvi Jingar wrote: > > Device drivers use get_device_system_crosststamp() to produce > > precise > > system/device cross-timestamps. The PHC clock and ALSA interfaces, > > for example, make the cross-timestamps available to user > > applications. > > On Intel platforms, get_device_system_crosststamp() requires a TSC > > value derived from ART (Always Running Timer) to compute the > > monotonic > > raw and realtime system timestamps. > > > > Starting with Intel Goldmont platforms, the PCIe root complex > > supports > > the PTM time sync protocol. PTM requires all timestamps to be in > > units > > of nanoseconds. The Intel root complex hardware propagates system > > time – > > derived from ART - in units of nanoseconds performing the > > conversion > > as follows: > > > > ART_NS = ART * 1e9 / <crystal frequency> > > > > When user software requests a cross-timestamp, the system > > timestamps > > (generally read from device registers) must be converted to TSC by > > the driver software as follows: > > > > TSC = ART_NS * TSC_KHZ / 1e6 > > > > This is valid when CPU feature flag X86_FEATURE_TSC_KNOWN_FREQ is > > set > > indicating the tsc_khz is derived from CPUID[15H]. Drivers should > > check that this flag is set before conversion to TSC is attempted. > > Clear and coherent changelog. Well done! > > > Changes from v1: > > > > * use existing frequency hardcode for platforms where > > CPUID[15H].ECX == 0 > > (v1 added redundant hardcode just for the ART.ns conversion) > > > > * use tsc_khz for TSC conversion, also requires driver to check > > X86_FEATURE_TSC_KNOWN_FREQ (v1 used CPUID[15H].ECX value directly) > > Maintainer lazyness request: Can you please put the changes paragraph > below > the --- seperator so it is discarded when the patch is extracted from > mail. It's not part of the changelog which goes into git. >
Sure. It has been changed in v3.
> > +struct system_counterval_t convert_art_ns_to_tsc(u64 art_ns) > > Can you please add kernel doc format function documentation which > explains > the calling conventions? >
kernel-do format documentation is added to v3.
> > +{ > > + u64 tmp, res, rem; > > + > > + rem = do_div(art_ns, USEC_PER_SEC); > > + > > + res = art_ns * tsc_khz; > > + tmp = rem * tsc_khz; > > + > > + do_div(tmp, USEC_PER_SEC); > > + res += tmp; > > + > > + return (struct system_counterval_t) {.cs = > > art_related_clocksource, > > + .cycles = res}; > > Definitely way better than the previous one. Good job! > > Thanks, > > tglx
Thank you, Rajvi
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