clocksource: sunxi: Timer fixes and cleanup
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Cover letter [linux-sunxi] [PATCH 0/8] clocksource: sunxi: Timer fixes and cleanup
Quoted 14 lines · On Fri, 28 Jun 2013 09:43:37 +0800 ?? <[email protected]> wrote:
Quoted 12 lines
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 readCNT64_LO_REGwhile latching is still in progress andCNT64_RL_ENbit inCNT64_CTRL_REGhas not changed to zero yet. I can imagine the following possible scenarios:
1. We read either the old staleCNT64_LO_REGvalue or the new correct value.
2. We read either the old staleCNT64_LO_REGvalue or the new correct value, or some random garbage.
3. The processor may deadlock, eat your dog, or do some other nasty thing.Quoted 1 line
In the case of 1, we probably can get away without using any spinlocks?
Quoted 10 lines
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.
Thanks for the clarification! It is very much appreciated.
So basically we get scenario 2, which still allows some optimizations compared to scenario 3. On single-core systems (Allwinner A10), it probably makes sense to avoid spinlocks overhead and just place the critical code between LDREX and STREX instructions (or whatever higher level wrappers are available for them). In the case of contention and reading non-latched garbage value from CNT64_LO_REG (or from CNT64_LO_REG/CNT64_HI_REG pair if we go 64-bit), this critical block of code can be just retried again until it eventually succeeds.
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Best regards, Siarhei Siamashka