Re: [PATCH 2/4] ACPI: PCC: Preserve shared memory signature in OpRegion handler

[Rafael J. Wysocki (Intel)]

On Tue, Jul 21, 2026 at 2:27 PM Sudeep Holla <sudeep.holla@xxxxxxxxxx> wrote:
>
> On Tue, Jul 21, 2026 at 07:43:53PM +0800, lihuisong (C) wrote:
> >
> > On 7/17/2026 4:10 PM, Sudeep Holla wrote:
> > > ACPI 6.3 introduced PCC OperationRegions. Section 5.5.2.4.7.3,
> > > "Declaring message fields within a PCC OperationRegion", states that,
> > > for all PCC subspace types, the PCC Operation Region pertains to the
> > > region of PCC subspace that succeeds the PCC signature.
> > >
> > > The PCC address space handler currently copies the OperationRegion
> > > buffer to and from the start of the PCC shared memory region. That can
> > > overwrite or expose the signature at byte offset 0, and it also misses
> > > the last 4 bytes of the actual PCC OperationRegion data.
> > >
> > > Offset OperationRegion copies by the size of the signature and reject
> > > regions that do not fit in the shared memory after that signature. AML
> > > that sizes the PCC OperationRegion to include the signature is not
> > > conforming to the PCC OperationRegion definition.
> > >
> > > Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> > > Fixes: 77e2a04745ff ("ACPI: PCC: Implement OperationRegion handler for the PCC Type 3 subtype")
> > > Signed-off-by: Sudeep Holla <sudeep.holla@xxxxxxxxxx>
> > > ---
> > The below commit description said the PCC operationRegion contains the
> > signature field.
> > "77e2a04745ff ("ACPI: PCC: Implement OperationRegion handler for the PCC
> > Type 3 subtype")"
> > And I find that the definition of this OperationRegion in ACPI spec 6.3 is
> > the same with spec 6.5 and 6.6.
> > That is, the original implementation already didn't comply with ACPI spec.
> >
> > Patch 1/4 also requires the change of platform AML about PCC
> > OperationRegion.
> > These modification may lead to the existed PCC OperationRegion method in
> > some drivers doesn't work.
> > Because the platform AML still contains signature field in PCC
> > OperationRegion method.
> > I don't know how many drivers in linux have used this method.
> > IMO, we at least need to reclarify this usage change and provide a new usage
> > example in the commit log like the original commit did.
> >
>
> Good point, I didn't realise I had wrong example in the original commit.
> I will add the example of correct usage in this change. Thanks for pointing
> it out.
>
> > Anyway, this patch also makes the driver more compliant with the
> > specifications.
> > So I agree.
> > Reviewed-by: Huisong Li <lihuisong@xxxxxxxxxx>
> >
>
> Thanks again for all the review.

I gather that there will be a v2 then.