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

[Scott Wood <scottwood@freescale.com>]

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;
>=20
> Are the three mftbu/l instructions guaranteed to be executed
> in order?

This is the architecturally defined instruction sequence for 32-bit, =20
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 =20
deviating from the instruction sequence called for by the erratum text, =20
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=