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.