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

[Sudeep Holla]

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.

-- 
Regards,
Sudeep