linux-kernel - Re: [PATCH v2] PCI: Do not attempt to set ExtTag for VFs

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Message-ID: <2d18db0c-185e-432c-8694-5dfb58c18cd1@linux.dev>
Date: Tue, 2 Dec 2025 10:16:55 -0800
From: "yanjun.zhu" <yanjun.zhu@...ux.dev>
To: Haakon Bugge <haakon.bugge@...cle.com>,
 Bjorn Helgaas <bhelgaas@...gle.com>, Sinan Kaya <okaya@...eaurora.org>
Cc: "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] PCI: Do not attempt to set ExtTag for VFs

On 12/1/25 11:56 PM, Haakon Bugge wrote:
> 
> 
>> On 12 Nov 2025, at 10:54, Håkon Bugge <Haakon.Bugge@...cle.com> wrote:
>>
>> The bit for enabling extended tags is Reserved and Preserved (RsvdP)
>> for VFs, according to PCIe r7.0 section 7.5.3.4 table 7.21.  Hence,
>> bail out early from pci_configure_extended_tags() if the device is a
>> VF.

enabling extended tags in PCIe r7.0 section 7.5.3.4 table 7.21 is as below:
"
Extended Tag Field Enable - This bit, in combination with the 10-Bit Tag 
Requester Enable bit and the 14-Bit Tag Requester Enable bit, determines 
how many Tag field bits a Requester is permitted to use for non-UIO 
Requests. This bit is not applicable to a Requester when generating UIO 
Requests. The following applies when the 10-Bit Tag Requester Enable bit 
and the 14-Bit Tag Requester Enable bit are both Clear. If the Extended 
Tag Field Enable bit is Set, the Function is permitted to use an 8-bit 
Tag field as a Requester. If the bit is Clear, the Function is 
restricted to using a 5-bit Tag field. See § Section 2.2.6.2 for 
required behavior when one or both of these larger-Tag Requester Enable 
bits are Set. If software changes the value of the Extended Tag Field 
Enable bit while the Function has outstanding Non-Posted Requests, the 
result is undefined. Functions that do not implement this capability 
hardwire this bit to 0b. Default value of this bit is implementation 
specific.
"

Thus,
Reviewed-by: Zhu Yanjun <yanjun.zhu@...ux.dev>

Zhu Yanjun

>>
>> Otherwise, we may see incorrect log messages such as:
>>
>>    kernel: pci 0000:af:00.2: enabling Extended Tags
>>
>> (af:00.2 is a VF)
>>
>> Fixes: 60db3a4d8cc9 ("PCI: Enable PCIe Extended Tags if supported")
>> Signed-off-by: Håkon Bugge <haakon.bugge@...cle.com>
> 
> A gentle ping on this one.
> 
> 
> Thxs, Håkon
> 
>>
>> ---
>>
>> v1 -> v2: Added ref to PCIe spec
>> ---
>> drivers/pci/probe.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>> index 0ce98e18b5a87..014017e15bcc8 100644
>> --- a/drivers/pci/probe.c
>> +++ b/drivers/pci/probe.c
>> @@ -2244,7 +2244,8 @@ int pci_configure_extended_tags(struct pci_dev *dev, void *ign)
>> u16 ctl;
>> int ret;
>>
>> - if (!pci_is_pcie(dev))
>> + /* PCI_EXP_DEVCTL_EXT_TAG is RsvdP in VFs */
>> + if (!pci_is_pcie(dev) || dev->is_virtfn)
>> return 0;
>>
>> ret = pcie_capability_read_dword(dev, PCI_EXP_DEVCAP, &cap);
>> -- 
>> 2.43.5
>>
>