Date: Mon, 11 Dec 2017 20:02:24 -0800
From: Jakub Kicinski <kubakici@wp.pl>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH] new byteorder primitives -
 ..._{replace,get}_bits()
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On Mon, 11 Dec 2017 15:54:22 +0000, Al Viro wrote:
> Essentially, it gives helpers for work with bitfields in fixed-endian.
> Suppose we have e.g. a little-endian 32bit value with fixed layout;
> expressing that as a bitfield would go like
> 	struct foo {
> 		unsigned foo:4;		/* bits 0..3 */
> 		unsigned :2;
> 		unsigned bar:12;	/* bits 6..17 */
> 		unsigned baz:14;	/* bits 18..31 */
> 	}
> Even for host-endian it doesn't work all that well - you end up with
> ifdefs in structure definition and generated code stinks.  For fixed-endian
> it gets really painful, and people tend to use explicit shift-and-mask
> kind of macros for accessing the fields (and often enough get the
> endianness conversions wrong, at that).  With these primitives
> 
> struct foo v		<=>	__le32 v
> v.foo = i ? 1 : 2	<=>	v = le32_replace_bits(v, i ? 1 : 2, 0, 4)
> f(4 + v.baz)		<=>	f(4 + le32_get_bits(v, 18, 14))

Looks very useful.  The [start bit, size] pair may not land itself
too nicely to creating defines, though.  Which is why in
include/linux/bitfield.h we tried to use a shifted mask and work
backwards from that single value what the start and size are.  commit
3e9b3112ec74 ("add basic register-field manipulation macros") has the
description.  Could a similar trick perhaps be applicable here?