clocksource/drivers/arc_timer: Add comments about locking while read GFRC

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: NEugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Acked-by: NVineet Gupta <vgupta@synopsys.com>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>

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);