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

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Message-ID: <A142D5A2-9EA0-4B1D-85A0-51C47656A0BA@fb.com>
Date:   Thu, 8 Sep 2022 22:41:48 +0000
From:   "Jonathan Zhang (Infra)" <jonzhang@...com>
To:     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



> On Sep 8, 2022, at 3:31 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> 
> !-------------------------------------------------------------------|
>  This Message Is From an External Sender
> 
> |-------------------------------------------------------------------!
> 
> 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.

Agreed. In this case, ACPI spec should be referred to.