Re: [PATCH v9 4/6] ACPI: HMAT: Fix handling of changes from ACPI 6.2 to ACPI 6.3

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Keith, author of 3accf7ae37a9 ("acpi/hmat: Parse and report
heterogeneous memory")]

On Fri, Aug 21, 2020 at 09:42:58AM +0100, Jonathan Cameron wrote:
> On Thu, 20 Aug 2020 17:21:29 -0500
> Bjorn Helgaas <helgaas@kernel.org> wrote:
> 
> > On Wed, Aug 19, 2020 at 10:51:09PM +0800, Jonathan Cameron wrote:
> > > In ACPI 6.3, the Memory Proximity Domain Attributes Structure
> > > changed substantially.  One of those changes was that the flag
> > > for "Memory Proximity Domain field is valid" was deprecated.
> > > 
> > > This was because the field "Proximity Domain for the Memory"
> > > became a required field and hence having a validity flag makes
> > > no sense.
> > > 
> > > So the correct logic is to always assume the field is there.
> > > Current code assumes it never is.
> > > 
> > > Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > > ---
> > >  drivers/acpi/numa/hmat.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/acpi/numa/hmat.c b/drivers/acpi/numa/hmat.c
> > > index 2c32cfb72370..07cfe50136e0 100644
> > > --- a/drivers/acpi/numa/hmat.c
> > > +++ b/drivers/acpi/numa/hmat.c
> > > @@ -424,7 +424,7 @@ static int __init hmat_parse_proximity_domain(union acpi_subtable_headers *heade
> > >  		pr_info("HMAT: Memory Flags:%04x Processor Domain:%u Memory Domain:%u\n",
> > >  			p->flags, p->processor_PD, p->memory_PD);
> > >  
> > > -	if (p->flags & ACPI_HMAT_MEMORY_PD_VALID && hmat_revision == 1) {
> > > +	if ((p->flags & ACPI_HMAT_MEMORY_PD_VALID && hmat_revision == 1) || hmat_revision == 2) {  
> > 
> > I hope/assume the spec is written in such a way that p->memory_PD is
> > required for any revision > 1?  So maybe this should be:
> > 
> >   if ((p->flags & ACPI_HMAT_MEMORY_PD_VALID && hmat_revision == 1) ||
> >       hmat_revision > 1) {

I should have said simply:

  if (hmat_revision == 1 && p->flags & ACPI_HMAT_MEMORY_PD_VALID)

We shouldn't even test p->flags for ACPI_HMAT_MEMORY_PD_VALID unless
we already know it's revision 1.

And unless there was a revision 0 of HMAT, there's no need to look for
hmat_revison > 1.

> Good point.  We have existing protections elsewhere against
> hmat_revision being anything other than 1 or 2, so we should aim to
> keep that in only one place.

I think the "Ignoring HMAT: Unknown revision" test in hmat_init(),
added by 3accf7ae37a9 ("acpi/hmat: Parse and report heterogeneous
memory"), is a mistake.

And I think hmat_normalize() has a similar mistake in that it tests
explicitly for hmat_revision == 2 when it should accept 2 AND anything
later.

We should assume that future spec revisions will be backwards
compatible.  Otherwise we're forced to make kernel changes when we
otherwise would not have to.

Bjorn