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

[Daniel Lezcano]

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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