[PATCH] powerpc/fsl-booke: Work around erratum A-006958

[Scott Wood]

On 07/15/2013 03:45:36 AM, David Laight wrote:
> > +#define MFTB(dest, scratch1, scratch2)	\
> > +90:	mftbu	scratch1;		\
> > +	mftbl	dest;			\
> > +	mftbu	scratch2;		\
> > +	cmpw	scratch1,scratch2;	\
> > +	bne	90b;			\
> > +	rldimi	dest,scratch1,32,0;
> 
> Are the three mftbu/l instructions guaranteed to be executed
> in order?

This is the architecturally defined instruction sequence for 32-bit,  
and is what the erratum writeup says to use.

> Also, if the high word changes, there is no need to loop.
> Just return the second value with a low word of zero
> (the returned count happened while the function was active).

That would be more complicated than looping.

> If the mftbx instructions are synchronizing ones (or need
> sequencing), then it might be possible to only have 2 of them
> by reading the 64bit value and the high 32bits.
> (The correct order depends on the exact definition of the errata!)

Again, I'm not sure how helpful that would be, and it would be  
deviating from the instruction sequence called for by the erratum text,  
which says to use the instruction sequence used on 32-bit.

> Another option is that the 64bit value returned by mftb is
> (presumably) only likely to be wrong when the low 32bits is
> very near zero (either side).
> A second mftb could be done in this case only with some logic
> to work out a valid result (I think):
>   High changed - 2nd high, low zero.
>   First low -ve - first value.
>   First low +ve - second value.

Likewise.

-Scott