linux-kernel - Re: [PATCH 09/15] cxl/acpi: Determine PCI host bridge's ACPI UID

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Message-ID: <631a6d394b4ef_166f2947c@dwillia2-xfh.jf.intel.com.notmuch>
Date:   Thu, 8 Sep 2022 15:31:21 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Jonathan Zhang (Infra)" <jonzhang@...com>,
        Dan Williams <dan.j.williams@...el.com>
CC:     Robert Richter <rrichter@....com>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "Ira Weiny" <ira.weiny@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>
Subject: Re: [PATCH 09/15] cxl/acpi: Determine PCI host bridge's ACPI UID

Jonathan Zhang (Infra) wrote:
> 
> 
> > On Sep 8, 2022, at 2:10 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> > 
> > !-------------------------------------------------------------------|
> >  This Message Is From an External Sender
> > 
> > |-------------------------------------------------------------------!
> > 
> > Jonathan Zhang (Infra) wrote:
> >> 
> >> 
> >>> On Aug 31, 2022, at 1:15 AM, Robert Richter <rrichter@....com> wrote:
> >>> 
> >>> The UID is needed to read the RCH's CEDT entry with the RCRB base
> >>> address. Determine the host's UID from its ACPI fw node.
> >>> 
> >>> Signed-off-by: Robert Richter <rrichter@....com>
> >>> ---
> >>> drivers/cxl/acpi.c | 12 ++++++++++++
> >>> 1 file changed, 12 insertions(+)
> >>> 
> >>> diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> >>> index f9cdf23a91a8..b3146b7ae922 100644
> >>> --- a/drivers/cxl/acpi.c
> >>> +++ b/drivers/cxl/acpi.c
> >>> @@ -368,8 +368,20 @@ struct pci_host_bridge *cxl_find_next_rch(struct pci_host_bridge *host)
> >>> static int __init cxl_restricted_host_probe(struct platform_device *pdev)
> >>> {
> >>> 	struct pci_host_bridge *host = NULL;
> >>> +	struct acpi_device *adev;
> >>> +	unsigned long long uid = ~0;
> >>> 
> >>> 	while ((host = cxl_find_next_rch(host)) != NULL) {
> >>> +		adev = ACPI_COMPANION(&host->dev);
> >>> +		if (!adev || !adev->pnp.unique_id ||
> >>> +			(kstrtoull(adev->pnp.unique_id, 10, &uid) < 0))
> >> The UID field holds 4 bytes of ASCII characters. kstrtoull() would fail
> >> if the UID field has value such as ‘CX03’.
> > 
> > The UID field is not 4 ASCII characters.
> > 
> > We went through this before in the original code in
> > drivers/cxl/acpi.c::add_host_bridge_dport().
> > 
> > The CEDT.CHBS defines _UID as an integer so use acpi_evaluate_integer()
> > to retrieve the UID to perform the comparison. I thought there was an
> > errata filed to clarify this, but it seems the current spec still just
> > says "value". The CFMWS also places _UID values in the target list,
> > those are also handled as integers.
> 
> ACPI 6.4 spec section 6.1.12 describes _UID, it says the return value is:
> An Integer or String containing the Unique ID.
> 
> In the BIOS I see, the _UIDs of PCIe devices hold ASCII characters (not NULL
> terminated). 

ASCII characters without NULL termination means that data can be
treated as binary data which is what current CFMWWS parsing code chooses
to do. I think a spec clarification is needed to make resolve the
ambiguity.