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(®, a_mask, a_val << a_shift);
Not quite:
clrsetbits_le32(®, 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