linux-kernel - Re: [PATCH] clocksource/drivers/pistachio: Fix wrong calculated clocksource read value

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Date:	Wed, 16 Dec 2015 09:33:40 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	Jisheng Zhang <jszhang@...vell.com>, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] clocksource/drivers/pistachio: Fix wrong calculated
 clocksource read value

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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