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

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Message-ID: <20150105134327.GA8156@localhost.localdomain>
Date:	Mon, 5 Jan 2015 14:43:27 +0100
From:	Richard Cochran <richardcochran@...il.com>
To:	David Laight <David.Laight@...LAB.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?

Thanks
Richard




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