Re: ERRATUM_858921 is broken on 5.15 kernel

[Marc Zyngier]

On Thu, 05 Jan 2023 13:33:48 +0000,
Yogesh Lal <quic_ylal@xxxxxxxxxxx> wrote:
> 
> Hi,
> 
> We are observing issue on A73 core where ERRATUM_858921 is broken.
> 
> On 5.15 kernel arch_timer_enable_workaround is set by reading
> arm64_858921_read_cntpct_el0 and arm64_858921_read_cntvct_el0 during
> timer register using following path.

Have you checked whether the issue is still present on 6.1?

> 
> arch_timer_enable_workaround->atomic_set(&timer_unstable_counter_workaround_in_use,
> 1);
> 
> [code snap]
> 564 static
> 565 void arch_timer_enable_workaround(const struct
> arch_timer_erratum_workaround *wa,
> 566                               bool local)
> 567 {
> 568     int i;
> 569
> 570     if (local) {
> 571 __this_cpu_write(timer_unstable_counter_workaround, wa);
> 572     } else {
> 573             for_each_possible_cpu(i)
> 574                     per_cpu(timer_unstable_counter_workaround, i) = wa;
> 575     }
> 576
> 577     if (wa->read_cntvct_el0 || wa->read_cntpct_el0)
> 578 atomic_set(&timer_unstable_counter_workaround_in_use, 1);
> 
> 
> and based on above workaround enablement , appropriate function to get
> counter is used.
> 
> 1008 static void __init arch_counter_register(unsigned type)
> 1009 {
> 1010     u64 start_count;
> 1011
> 1012     /* Register the CP15 based counter if we have one */
> 1013     if (type & ARCH_TIMER_TYPE_CP15) {
> 1014         u64 (*rd)(void);
> 1015
> 1016         if ((IS_ENABLED(CONFIG_ARM64) && !is_hyp_mode_available()) ||
> 1017             arch_timer_uses_ppi == ARCH_TIMER_VIRT_PPI) {
> 1018             if (arch_timer_counter_has_wa())
> 1019                 rd = arch_counter_get_cntvct_stable;
> 1020             else
> 1021                 rd = arch_counter_get_cntvct;
> 1022         } else {
> 1023             if (arch_timer_counter_has_wa())
> 1024                 rd = arch_counter_get_cntpct_stable;
> 1025             else
> 1026                 rd = arch_counter_get_cntpct;
> 1027         }
> [snap]
> 1043     /* 56 bits minimum, so we assume worst case rollover */
> 1044     sched_clock_register(arch_timer_read_counter, 56, arch_timer_rate);
> 
> 
> As our boot cores are not impacted by errata sched_clock_register()
> will register !arch_timer_counter_has_wa() callback.
> 
> Now when errata impacted core boots up and sched_clock_register
> already register will !arch_timer_counter_has_wa() path.
> As sched_clock_register is not per_cpu bases so
> arch_timer_read_counter will always point to
> !arch_timer_counter_has_wa() function calls.

Please try the following hack, only compile tested as I do not have
access to any affected HW, and report whether this solves your issue
or not. Note that this is based on 6.2-rc2.

Thanks,

	M.

diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
index e09d4427f604..a7cf0a2c86d3 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -230,6 +230,28 @@ static u64 arch_counter_read_cc(const struct cyclecounter *cc)
 	return arch_timer_read_counter();
 }
 
+static bool arch_timer_counter_has_wa(void);
+
+static u64 (*arch_counter_get_read_fn(void))(void)
+{
+	u64 (*rd)(void);
+
+	if ((IS_ENABLED(CONFIG_ARM64) && !is_hyp_mode_available()) ||
+	    arch_timer_uses_ppi == ARCH_TIMER_VIRT_PPI) {
+		if (arch_timer_counter_has_wa())
+			rd = arch_counter_get_cntvct_stable;
+		else
+			rd = arch_counter_get_cntvct;
+	} else {
+		if (arch_timer_counter_has_wa())
+			rd = arch_counter_get_cntpct_stable;
+		else
+			rd = arch_counter_get_cntpct;
+	}
+
+	return rd;
+}
+
 static struct clocksource clocksource_counter = {
 	.name	= "arch_sys_counter",
 	.id	= CSID_ARM_ARCH_COUNTER,
@@ -571,8 +593,10 @@ void arch_timer_enable_workaround(const struct arch_timer_erratum_workaround *wa
 			per_cpu(timer_unstable_counter_workaround, i) = wa;
 	}
 
-	if (wa->read_cntvct_el0 || wa->read_cntpct_el0)
+	if (wa->read_cntvct_el0 || wa->read_cntpct_el0) {
 		atomic_set(&timer_unstable_counter_workaround_in_use, 1);
+		arch_timer_read_counter = arch_counter_get_read_fn();
+	}
 
 	/*
 	 * Don't use the vdso fastpath if errata require using the
@@ -641,7 +665,7 @@ static bool arch_timer_counter_has_wa(void)
 #else
 #define arch_timer_check_ool_workaround(t,a)		do { } while(0)
 #define arch_timer_this_cpu_has_cntvct_wa()		({false;})
-#define arch_timer_counter_has_wa()			({false;})
+static bool arch_timer_counter_has_wa(void)		{ return false; }
 #endif /* CONFIG_ARM_ARCH_TIMER_OOL_WORKAROUND */
 
 static __always_inline irqreturn_t timer_handler(const int access,
@@ -1079,22 +1103,7 @@ static void __init arch_counter_register(unsigned type)
 
 	/* Register the CP15 based counter if we have one */
 	if (type & ARCH_TIMER_TYPE_CP15) {
-		u64 (*rd)(void);
-
-		if ((IS_ENABLED(CONFIG_ARM64) && !is_hyp_mode_available()) ||
-		    arch_timer_uses_ppi == ARCH_TIMER_VIRT_PPI) {
-			if (arch_timer_counter_has_wa())
-				rd = arch_counter_get_cntvct_stable;
-			else
-				rd = arch_counter_get_cntvct;
-		} else {
-			if (arch_timer_counter_has_wa())
-				rd = arch_counter_get_cntpct_stable;
-			else
-				rd = arch_counter_get_cntpct;
-		}
-
-		arch_timer_read_counter = rd;
+		arch_timer_read_counter = arch_counter_get_read_fn();
 		clocksource_counter.vdso_clock_mode = vdso_default;
 	} else {
 		arch_timer_read_counter = arch_counter_get_cntvct_mem;

-- 
Without deviation from the norm, progress is not possible.