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Subject[PATCH 3/7] clocksource/drivers/arc_timer: Add comments about locking while read GFRC
Date
From: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>

This came to light in some internal discussions and it is nice to have
this documented rather than digging up the PRM (Prog Ref Manual) again.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Acked-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
---
drivers/clocksource/arc_timer.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)

diff --git a/drivers/clocksource/arc_timer.c b/drivers/clocksource/arc_timer.c
index 471b428..20da9b1 100644
--- a/drivers/clocksource/arc_timer.c
+++ b/drivers/clocksource/arc_timer.c
@@ -61,6 +61,20 @@ static u64 arc_read_gfrc(struct clocksource *cs)
unsigned long flags;
u32 l, h;

+ /*
+ * From a programming model pov, there seems to be just one instance of
+ * MCIP_CMD/MCIP_READBACK however micro-architecturally there's
+ * an instance PER ARC CORE (not per cluster), and there are dedicated
+ * hardware decode logic (per core) inside ARConnect to handle
+ * simultaneous read/write accesses from cores via those two registers.
+ * So several concurrent commands to ARConnect are OK if they are
+ * trying to access two different sub-components (like GFRC,
+ * inter-core interrupt, etc...). HW also supports simultaneously
+ * accessing GFRC by multiple cores.
+ * That's why it is safe to disable hard interrupts on the local CPU
+ * before access to GFRC instead of taking global MCIP spinlock
+ * defined in arch/arc/kernel/mcip.c
+ */
local_irq_save(flags);

__mcip_cmd(CMD_GFRC_READ_LO, 0);
--
2.7.4
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