Hi Thomas, On Fri, Jun 28, 2013 at 04:02:23PM +0200, Thomas Gleixner wrote: > On Fri, 28 Jun 2013, 张猛 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. > > > > I really wonder why we're trying to use that timer. AFAICT the A10 has > another six 32bit timers which do not have this restriction and the > clocksoure/sched_clock implementation works nicely with 32 bits. So > what's the point of using that 64 bit counter if it's horrible to > access?
Yes, you're right. I actually wanted at first not to use a timer for this since we had a counter to do just that. But that's true that actually using a timer would make the code simpler and presumably faster as well. I'll change this in the v2. Thanks, Maxime -- Maxime Ripard, Free Electrons Embedded Linux, Kernel and Android engineering http://free-electrons.com
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