| Message ID | 20180417155232.4989-1-Eugeniy.Paltsev@synopsys.com |
|---|---|
| State | New |
| Headers | show |
| Series | clocksource: arc_timer: add comments about locking while read GFRC | expand |
On 04/17/2018 08:52 AM, Eugeniy Paltsev wrote: > 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> Minor nits below, otherwise LGTM ! Acked-by: Vineet Gupta <vgupta@synopsys.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 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..... > + * 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 s/support/supports/ > + * 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);
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);
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> --- drivers/clocksource/arc_timer.c | 13 +++++++++++++ 1 file changed, 13 insertions(+)