[linux-sunxi] [PATCH 0/8] clocksource: sunxi: Timer fixes and cleanup

[siarhei.siamashka@gmail.com (Siarhei Siamashka)]

On Fri, 28 Jun 2013 09:43:37 +0800
?? <kevin@allwinnertech.com> wrote:

> > The A10 manual from http://free-electrons.com/~maxime/pub/datasheet/
> > does not seem to contain any details about what bad things may happen
> > if we try to read CNT64_LO_REG while latching is still in progress and
> > CNT64_RL_EN bit in CNT64_CTRL_REG has not changed to zero yet.
> > I can imagine the following possible scenarios:
> >  1. We read either the old stale CNT64_LO_REG value or the new
> >     correct value.
> >  2. We read either the old stale CNT64_LO_REG value or the new
> >     correct value, or some random garbage.
> >  3. The processor may deadlock, eat your dog, or do some other
> >     nasty thing.
> >
> > In the case of 1, we probably can get away without using any spinlocks?
> 
> About the 64bits counter, the latch bit is needed because of the
> asynchronous circuit. The internal circuit of 64bits counter is
> working under 24Mhz clock, and CNT_LO/HI is read with APB clock.
> So the clock synchronize is needed. The function of the latch is
> synchronous the 64bits counter from 24Mhz clock domain to APB clock
> domain. So, if the latch is not completely, value of the CNT_LO/HI
> maybe a random value, because some bits are latched, but others are
> not. So, the CNT_LO/HI should be read after latch is completely.
> The latch just takes 3 cycles of 24Mhz clock, the time is nearly
> 0.125 micro-second.  

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Ugh, just spotted this at the bottom.

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-- 
Best regards,
Siarhei Siamashka