linux-kernel - RE: [PATCH net-next 1/7] timecounter: provide a macro to initialize the cyclecounter mask field.

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Date:	Mon, 5 Jan 2015 13:53:51 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Richard Cochran' <richardcochran@...il.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	"John Stultz" <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: RE: [PATCH net-next 1/7] timecounter: provide a macro to initialize
 the cyclecounter mask field.

> On Mon, Jan 05, 2015 at 01:20:57PM +0000, David Laight wrote:
> > > +/* simplify initialization of mask field */
> > > +#define CYCLECOUNTER_MASK(bits) (cycle_t)((bits) < 64 ? ((1ULL<<(bits))-1) : -1)
> >
> > That has me chasing through the C integer promotion rules.
> > Better might be:
> > 	((bits) < 64 ? (1ULL << (bits)) - 1 : (((1ULL << 63) - 1) << 1) + 1)
> > I actually suspect there is a standard definition somewhere?
> 
> This is an exact copy of CLOCKSOURCE_MASK, and if wrong, then both are
> wrong.  In any case, I can't see any issue here. Is not
> 
> 	(some_int_type) -1
> 
> always equal to
> 
> 	0xf...(width of type)
> 
> for all integer types, when using 2s compliment?

As I said, it leaves me chasing through the promotion rules (which I
probably know if I actually think hard enough).

Thinking... ~0ULL would be nice and simple and correct.

	David

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