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

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Message-ID: <56719011.4080500@linaro.org>
Date:	Wed, 16 Dec 2015 17:23:45 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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 12/16/2015 11:38 AM, Russell King - ARM Linux wrote:
> On Wed, Dec 16, 2015 at 11:32:17AM +0100, Daniel Lezcano wrote:
>> 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.
>
> That's not my point.  Whether you do:
>
> 	~(cycle_t)readl(...)
>
> or
>
> 	(cycle_t)~readl(...)
>
> is irrelevant - the result is the same as far as the core code is
> concerned as it doesn't care about the higher order bits.
>
> The only thing about which should be done is really which is faster
> in the general case, since this is a fast path in the time keeping
> code.

Ah, ok. Yes, I agree.


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