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

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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>
This commit is contained in:
Eugeniy Paltsev 2018-04-19 18:53:05 +03:00 committed by Daniel Lezcano
parent 3ca5768de8
commit 6bd9549d8a
1 changed files with 14 additions and 0 deletions
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14
drivers/clocksource/arc_timer.c
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@ -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);