[Minios-devel] [UNIKRAFT PATCHv6 13/37] plat/common: Add counter workaround for Cortex-A73 erratum 858921

[Wei Chen <wei.chen@xxxxxxx>]

The errata #858921 describes that Cortex-A73 (r0p0 - r0p2)
counter read can return a wrong value when the counter crosses
a 32bit boundary, but newer Cortex-A73 are not affected.

The workaround involves performing the read twice, compare
bit[32] of the two read values. If bit[32] is different,
keep the first value, otherwise keep the second value.

Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
---
 arch/arm/arm64/Config.uk |  8 ++++++++
 plat/common/arm/time.c   | 20 ++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/arch/arm/arm64/Config.uk b/arch/arm/arm64/Config.uk
index 7797516..8c20e68 100644
--- a/arch/arm/arm64/Config.uk
+++ b/arch/arm/arm64/Config.uk
@@ -46,3 +46,11 @@ config MARCH_ARM64_CORTEXA75
                Compile for Armv8.2 Cortex-A75 (and compatible) CPUs
 
 endchoice
+
+config ARM64_ERRATUM_858921
+       bool "Workaround for Cortex-A73 erratum 858921"
+       default y
+       help
+         This option enables a workaround for Cortex-A73 (r0p0 - r0p2),
+         whose counter may return a wrong value when the counter crosses
+         a 32-bit boundary. The newer Cortex-A73 are not affected.
diff --git a/plat/common/arm/time.c b/plat/common/arm/time.c
index a84e1a5..93b0521 100644
--- a/plat/common/arm/time.c
+++ b/plat/common/arm/time.c
@@ -63,10 +63,30 @@ static inline uint32_t get_counter_frequency(void)
        return SYSREG_READ32(cntfrq_el0);
 }
 
+#ifdef CONFIG_ARM64_ERRATUM_858921
+/*
+ * The errata #858921 describes that Cortex-A73 (r0p0 - r0p2) counter
+ * read can return a wrong value when the counter crosses a 32bit boundary.
+ * But newer Cortex-A73 are not affected.
+ *
+ * The workaround involves performing the read twice, compare bit[32] of
+ * the two read values. If bit[32] is different, keep the first value,
+ * otherwise keep the second value.
+ */
+static uint64_t read_virtual_count(void)
+{
+    uint64_t val_1st, val_2nd;
+
+    val_1st = SYSREG_READ64(cntvct_el0);
+    val_2nd = SYSREG_READ64(cntvct_el0);
+    return (((val_1st ^ val_2nd) >> 32) & 1) ? val_1st : val_2nd;
+}
+#else
 static inline uint64_t read_virtual_count(void)
 {
        return SYSREG_READ64(cntvct_el0);
 }
+#endif
 
 /*
  * monotonic_clock(): returns # of nanoseconds passed since
-- 
2.17.1


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