[PATCH] drivers/clocksource/arm_arch_timer: Tighten Allwinner arch timer workaround

[Ilya Dikariev]

As we know, the Allwinner A64 SoC has a buggy RCU time unit. The
workaround named UNKNOWN1 was not sufficient to cover some more buggy
bunches of this SoC. This workaround diminish the mask to 8 bits instead
of 9.

An example run of timer test tool https://github.com/smaeul/timer-tools
on PinePhone device (owns the A64 SoC) gives following result on a non
patched kernel (cut off):

Running parallel counter test...
0: Failed after 5507 reads (0.003578 s)
0: 0x0000000c8272cbf1 -> 0x0000000c8272ccff -> 0x0000000c8272cc0e (     0.011 ms)
2: Failed after 14518 reads (0.009248 s)
2: 0x0000000c827513f1 -> 0x0000000c82751300 -> 0x0000000c8275140e (    -0.010 ms)
3: Failed after 14112 reads (0.008730 s)
3: 0x0000000c8274f3f2 -> 0x0000000c8274f300 -> 0x0000000c8274f40d (    -0.010 ms)
1: Failed after 12030 reads (0.008409 s)
1: 0x0000000c8274abf1 -> 0x0000000c8274acff -> 0x0000000c8274ac0f (     0.011 ms)
1: 0x0000000c827759f2 -> 0x0000000c82775aff -> 0x0000000c82775a0e (     0.011 ms)
0: 0x0000000c8277a9f2 -> 0x0000000c8277aaff -> 0x0000000c8277aa0d (     0.011 ms)
2: 0x0000000c8278f3f1 -> 0x0000000c8278f300 -> 0x0000000c8278f40e (    -0.010 ms)
0: 0x0000000c82785ff2 -> 0x0000000c82784300 -> 0x0000000c8278600d (    -0.309 ms)

After the proposed patch applied the test runs
correctly (~2 hours of testing with a tool above without fails)

Signed-off-by: Ilya Dikariev <dikarill@xxxxxxx>
---
 drivers/clocksource/arm_arch_timer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
index a7ff77550e17..3019faa263f5 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -371,7 +371,7 @@ static u64 notrace arm64_858921_read_cntvct_el0(void)
 	do {								\
 		_val = read_sysreg(reg);				\
 		_retries--;						\
-	} while (((_val + 1) & GENMASK(8, 0)) <= 1 && _retries);	\
+	} while (((_val + 1) & GENMASK(7, 0)) <= 1 && _retries);	\
 									\
 	WARN_ON_ONCE(!_retries);					\
 	_val;								\
-- 
2.25.1