Messages in this thread | | | Subject | Re: [PATCH] clocksource/drivers/pistachio: Fix wrong calculated clocksource read value | From | Daniel Lezcano <> | Date | Wed, 16 Dec 2015 11:32:17 +0100 |
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On 12/16/2015 10:33 AM, Russell King - ARM Linux wrote: > On Wed, Dec 16, 2015 at 10:21:55AM +0100, Daniel Lezcano wrote: >> On 12/16/2015 08:36 AM, Jisheng Zhang wrote: >>> And in fact, clocksource_mmio_readw_down() also has similar issue, but it masks >>> with c->mask before return, the c->mask is less than 32 bit (because the >>> clocksource_mmio_init think number of valid bits > 32 or < 16 is invalid.) >>> the higher 32 bits are masked off, so we never saw such issue. But we'd better >>> to fix that, what's your opinion? >> >> I think we should have a look to this portion closely. > > There is no need to return more bits than are specified. If you have > a N-bit counter, then the high (64-N)-bits can be any value, because: > > static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask) > { > return (now - last) & mask; > } > > where 'now' is the current value returned from the clock source read > function, 'last' is a previously returned value, and 'mask' is the > bit mask. This has the effect of ignoring the high order bits.
I think this approach is perfectly sane. When I said we should look at this portion closely, I meant we should double check the bitwise-nor order regarding the explicit cast. The clocksource's mask makes sense and must stay untouched.
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