Messages in this thread Patch in this message | | | From | Rajvi Jingar <> | Subject | [PATCH v2] x86/tsc: Convert ART in nanoseconds to TSC. | Date | Tue, 6 Mar 2018 08:47:29 -0800 |
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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.
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)
Signed-off-by: Rajvi Jingar <rajvi.jingar@intel.com> Suggested-by: Christopher S. Hall <christopher.s.hall@intel.com> --- arch/x86/include/asm/tsc.h | 1 + arch/x86/kernel/tsc.c | 18 ++++++++++++++++++ 2 files changed, 19 insertions(+)
diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h index cf5d53c..2701d22 100644 --- a/arch/x86/include/asm/tsc.h +++ b/arch/x86/include/asm/tsc.h @@ -31,6 +31,7 @@ static inline cycles_t get_cycles(void) } extern struct system_counterval_t convert_art_to_tsc(u64 art); +extern struct system_counterval_t convert_art_ns_to_tsc(u64 art_ns); extern void tsc_early_delay_calibrate(void); extern void tsc_init(void); diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index fb43027..f07f3bd 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -1179,6 +1179,24 @@ struct system_counterval_t convert_art_to_tsc(u64 art) } EXPORT_SYMBOL(convert_art_to_tsc); +struct system_counterval_t convert_art_ns_to_tsc(u64 art_ns) +{ + 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}; +} +EXPORT_SYMBOL(convert_art_ns_to_tsc); + + static void tsc_refine_calibration_work(struct work_struct *work); static DECLARE_DELAYED_WORK(tsc_irqwork, tsc_refine_calibration_work); /** -- 2.7.4
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