From mboxrd@z Thu Jan  1 00:00:00 1970
From: Joe Perches <joe@perches.com>
Subject: Re: [PATCH v2 3/9] bitops: Introduce a more generic BITMASK macro
Date: Thu, 17 Oct 2013 01:32:30 -0700
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To: Chen Gong <gong.chen@linux.intel.com>
Cc: tony.luck@intel.com, bp@alien8.de, naveen.n.rao@linux.vnet.ibm.com, m.chehab@samsung.com, arozansk@redhat.com, linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org, Thomas Winischhofer <thomas@winischhofer.net>, Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>, Tomi Valkeinen <tomi.valkeinen@ti.com>

On Thu, 2013-10-17 at 03:38 -0400, Chen Gong wrote:
> the point is we can use GENMASK like GENMASK(end_bit, start_bit) but
> we don't know the value of end_bit/start_bit at compile-time.

True.

The BUILD_BUG_ON idea is just to avoid people using
	GENMASK(1, 2)
instead of
	GENMASK(2, 1)

#define GENMASK(h, l)						\
({								\
	BUILD_BUG_ON(__builtin_constant_p(l) &&			\
		     __builtin_constant_p(h) &&			\
		     (l) > (h));				\
	(((U32_C(1) << ((h) - (l) + 1)) - 1) << (l));		\
})