From mboxrd@z Thu Jan  1 00:00:00 1970
From: linux@arm.linux.org.uk (Russell King - ARM Linux)
Date: Wed, 16 Dec 2015 09:33:40 +0000
Subject: [PATCH] clocksource/drivers/pistachio: Fix wrong calculated
 clocksource read value
In-Reply-To: <56712D33.5080009@linaro.org>
References: <1448466169-5230-1-git-send-email-jszhang@marvell.com>
 <56707F32.3030405@linaro.org> <20151216151125.1e91b4f4@xhacker>
 <20151216152807.23491eee@xhacker> <20151216153609.0f09f941@xhacker>
 <56712D33.5080009@linaro.org>
Message-ID: <20151216093340.GD8644@n2100.arm.linux.org.uk>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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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