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    SubjectRe: [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@intel.com> wrote:
    > >
    > > !-------------------------------------------------------------------|
    > > This Message Is From an External Sender
    > >
    > > |-------------------------------------------------------------------!
    > >
    > > Jonathan Zhang (Infra) wrote:
    > >>
    > >>
    > >>> On Aug 31, 2022, at 1:15 AM, Robert Richter <rrichter@amd.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@amd.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.

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