Messages in this thread | | | Date | Wed, 16 Dec 2015 09:33:40 +0000 | From | Russell King - ARM Linux <> | Subject | Re: [PATCH] clocksource/drivers/pistachio: Fix wrong calculated clocksource read value |
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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.
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