[U-Boot] [PATCH v3 02/10] armv7: add miscellaneous utility macros

Graeme Russ
2011-06-16T11:07:53Z

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