Re: [PATCH] clocksource/drivers/pistachio: Fix wrong calculated clocksource read value
From: Daniel Lezcano
Date: Wed Dec 16 2015 - 05:32:20 EST
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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