linux-kernel - [PATCH] clocksource: arc_timer: add comments about locking while read GFRC

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Date:   Tue, 17 Apr 2018 18:52:32 +0300
From:   Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
To:     linux-snps-arc@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org,
        Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
Subject: [PATCH] clocksource: 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: Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
---
 drivers/clocksource/arc_timer.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/clocksource/arc_timer.c b/drivers/clocksource/arc_timer.c
index 4927355f9cbe..b594c373debc 100644
--- a/drivers/clocksource/arc_timer.c
+++ b/drivers/clocksource/arc_timer.c
@@ -61,6 +61,19 @@ static u64 arc_read_gfrc(struct clocksource *cs)
 	unsigned long flags;
 	u32 l, h;
 
+	/*
+	 * MCIP_CMD/MCIP_READBACK registers are allocated
+	 * 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 support 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);
-- 
2.14.3