[RFC PATCH v1 03/20] arch: x86: Add ART support function to tsc code

[lakshmi . sowjanya . d]

From: Christopher Hall <christopher.s.hall@xxxxxxxxx>

Add a function to 'read' the ART(Always Running Timer) value using
TSC(Time Stamp Capture) conversion.

The Intel PMC Timed I/O driver requires the current ART value to
test for rollover.

Signed-off-by: Christopher Hall <christopher.s.hall@xxxxxxxxx>
Signed-off-by: Tamal Saha <tamal.saha@xxxxxxxxx>
Co-developed-by: Lakshmi Sowjanya D <lakshmi.sowjanya.d@xxxxxxxxx>
Signed-off-by: Lakshmi Sowjanya D <lakshmi.sowjanya.d@xxxxxxxxx>
Reviewed-by: Mark Gross <mgross@xxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/tsc.h |  1 +
 arch/x86/kernel/tsc.c      | 20 ++++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h
index 01a300a9700b..a50b0102e5c1 100644
--- a/arch/x86/include/asm/tsc.h
+++ b/arch/x86/include/asm/tsc.h
@@ -28,6 +28,7 @@ static inline cycles_t get_cycles(void)
 	return rdtsc();
 }
 
+extern u64 read_art_time(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);
 
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 2e076a459a0c..bbab6cf1a73b 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1230,6 +1230,26 @@ int unsynchronized_tsc(void)
 	return 0;
 }
 
+/*
+ * Converts the current TSC to the current ART value using conversion
+ * factors discovered by detect_art()
+ */
+u64 read_art_time(void)
+{
+	u64 tsc, tmp, res, rem;
+
+	tsc = read_tsc(NULL) - art_to_tsc_offset;
+	rem = do_div(tsc, art_to_tsc_numerator);
+
+	res = tsc * art_to_tsc_denominator;
+	tmp = rem * art_to_tsc_denominator;
+
+	do_div(tmp, art_to_tsc_numerator);
+
+	return res + tmp;
+}
+EXPORT_SYMBOL(read_art_time);
+
 /*
  * Convert ART to TSC given numerator/denominator found in detect_art()
  */
-- 
2.17.1