From mboxrd@z Thu Jan  1 00:00:00 1970
From: Graeme Russ <graeme.russ@gmail.com>
Date: Thu, 16 Jun 2011 21:07:53 +1000
Subject: [U-Boot] [PATCH v3 02/10] armv7: add miscellaneous utility
	macros
In-Reply-To: <20110615125150.C7F7E138EF1F@gemini.denx.de>
References: <1299589658-30896-1-git-send-email-aneesh@ti.com>
	<1305202276-27784-3-git-send-email-aneesh@ti.com>
	<20110515184421.D7AB91491B06@gemini.denx.de>
	<4DD13DA3.3030505@ti.com> <4DECF8DA.9030806@ti.com>
	<20110606185046.BB8991736815@gemini.denx.de>
	<4DEDE8D9.7030306@ti.com>
	<20110607103923.7E1CC1B993A8@gemini.denx.de>
	<4DEE161B.2050402@ti.com>
	<20110607154028.D6540195494F@gemini.denx.de>
	<4DEF62A6.7060706@ti.com>
	<20110608214124.849821B993A8@gemini.denx.de>
	<4DF71FBF.6030408@ti.com>
	<20110614105115.01D6611EEDAF@gemini.denx.de>
	<4DF7488A.6000909@ti.com>
	<20110614135335.A750811EEDAF@gemini.denx.de>
	<4DF871E3.8080307@ti.com>
	<20110615092014.F080219E5AC1@gemini.denx.de>
	<4DF89102.9040508@ti.com>
	<20110615120447.9EEC4655C6@gemini.denx.de>
	<4DF8A8CF.5000308@gmail.com>
	<20110615125150.C7F7E138EF1F@gemini.denx.de>
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On 15/06/11 22:51, Wolfgang Denk wrote:
> Dear Graeme Russ,
> 
> In message <4DF8A8CF.5000308@gmail.com> you wrote:
>>
>> And to set the value then you have:
>>
>> 	reg &= ~a_mask;				/* Clear a_val */
>> 	reg |= (a_val << a_shift) & a_mask;	/* Set new a_val */
> 
> This could be done using
> 
> 	clrsetbits_le32(&reg, a_mask, a_val << a_shift);

Not quite:

	clrsetbits_le32(&reg, a_mask, (a_val << a_shift) & a_mask);

is equivalent except that, as already pointed out, clrsetbits and friends:

 a) Are not portable because only ARM and PPC define them which makes
    them, by definition, non-standard
 b) Each invocation results in a read barrier plus a write barrier
 c) If the hardware register is sensitive to partial updates (i.e. requires
    all bit-fields to be updated in on operation) this requires a read into
    a local variable, calls to clrsetbits against that variable and finally
    a write-back - Lots of memory barriers

I know I'm going over old ground, and it's not that I am against
clrsetbits, it's just good to know the limitations up-front. So remember,
clrsetbits is not platform independent, prevents the compiler from
optimising and will most likely impose a performance hit

Regards,

Graeme