arm: timer: sunxi: add Allwinner timer erratum workaround

[u-boot] / arch / arm / cpu / armv8 / generic_timer.c

diff --git a/arch/arm/cpu/armv8/generic_timer.c b/arch/arm/cpu/armv8/generic_timer.c
index 3d04fde6501ee6f503f94ec048d84f8c77b700aa..c1706dcec18cec3c856c19002f8d3f7eb28aae9c 100644 (file)
--- a/arch/arm/cpu/armv8/generic_timer.c
+++ b/arch/arm/cpu/armv8/generic_timer.c
@@ -46,6 +46,30 @@ unsigned long timer_read_counter(void)
 
        return cntpct;
 }
+#elif CONFIG_SUNXI_A64_TIMER_ERRATUM
+/*
+ * This erratum sometimes flips the lower 11 bits of the counter value
+ * to all 0's or all 1's, leading to jumps forwards or backwards.
+ * Backwards jumps might be interpreted all roll-overs and be treated as
+ * huge jumps forward.
+ * The workaround is to check whether the lower 11 bits of the counter are
+ * all 0 or all 1, then discard this value and read again.
+ * This occasionally discards valid values, but will catch all erroneous
+ * reads and fixes the problem reliably. Also this mostly requires only a
+ * single read, so does not have any significant overhead.
+ * The algorithm was conceived by Samuel Holland.
+ */
+unsigned long timer_read_counter(void)
+{
+       unsigned long cntpct;
+
+       isb();
+       do {
+               asm volatile("mrs %0, cntpct_el0" : "=r" (cntpct));
+       } while (((cntpct + 1) & GENMASK(10, 0)) <= 1);
+
+       return cntpct;
+}
 #else
 /*
  * timer_read_counter() using the Arm Generic Timer (aka arch timer).