linux-kernel - Re: [PATCH v2 3/9] bitops: Introduce a more generic BITMASK macro

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Message-ID: <1382033611.22110.131.camel@joe-AO722>
Date:	Thu, 17 Oct 2013 11:13:31 -0700
From:	Joe Perches <joe@...ches.com>
To:	Tony Luck <tony.luck@...il.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	Chen Gong <gong.chen@...ux.intel.com>,
	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
	m.chehab@...sung.com, arozansk@...hat.com,
	linux-acpi <linux-acpi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Winischhofer <thomas@...ischhofer.net>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>
Subject: Re: [PATCH v2 3/9] bitops: Introduce a more generic BITMASK macro

On Thu, 2013-10-17 at 09:10 -0700, Tony Luck wrote:
> On Thu, Oct 17, 2013 at 1:55 AM, Joe Perches <joe@...ches.com> wrote:
> > It's cost free to add the BUILD_BUG_ON
> > and perhaps you underestimate the runtime
> > bug checking effort,
> 
> This looks OK to me.  Gong: This doesn't stop people from using variables
> as arguments ... they just won't get a check for (h) < (l).

Another possibility is to swap high and low if necessary and
maybe warn when either is negative or too large for the bit width.

#define GENMASK(h, l)							\
({									\
	size_t high = h;						\
	size_t low = l;							\
	BUILD_BUG_ON(__builtin_constant_p(l) &&				\
		     __builtin_constant_p(h) &&				\
		     (l) > (h));					\
	BUILD_BUG_ON(__builtin_constant_p(l) &&				\
		     __builtin_constant_p(h) &&				\
		     ((l) >= BITS_PER_LONG ||				\
		      (h) >= BITS_PER_LONG));				\
	WARN_ONCE((!__builtin_constant_p(l) &&				\
		   ((l) < 0 || (l) > BITS_PER_LONG)) ||			\
		  (!__builtin_constant_p(h) &&				\
		   ((h) < 0 || (h) > BITS_PER_LONG)) ||			\
		  (l) > (h),						\
		  "GENMASK: invalid mask values: l: %u, h: %d\n",	\
		  (l), (h));						\
	if (low > high)							\
		swap(low, high);					\
	(((U32_C(1) << (high - low + 1)) - 1) << low);			\
})

And maybe this should be renamed something like
#define BIT_MASK_RANGE(h, l)
or
#define BIT_MASK_SHIFTED(h, l)


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