Re: [PATCH v3] PCI: vmd: Handle BUS_RESTRICT_CFG value 3 for Arrow Lake-HX

[Lin Mohan <linmhwork@outlook.com>]

> On Jul 21, 2026, at 06:47, Bjorn Helgaas <helgaas@kernel.org> wrote:
> 
> On Sun, Jun 28, 2026 at 04:34:50PM +0200, Ali Alaei wrote:
>> On Intel Arrow Lake-HX systems (e.g. Core Ultra 9 275HX on Acer Predator
>> PH16-73), the VMD controller reports BUS_RESTRICT_CFG = 3 in the VMCONFIG
>> register. The existing switch statement only handled values 0, 1, and 2,
>> causing vmd_get_bus_number_start() to return -ENODEV and aborting the
>> entire VMD probe. This leaves NVMe drives behind the VMD controller
>> invisible to the kernel.
>> 
>> Hardware registers (VMCAP/VMCONFIG at offsets 0x40/0x44):
>>  VMD 0000:00:0e.0 (8086:ad0b): VMCAP=0x000f, VMCONFIG=0x03b8
>>  BUS_RESTRICT_CFG(0x03b8) = (0x03b8 >> 8) & 0x3 = 3
>> 
>> Add cfg=3 as a fallthrough to cfg=2, setting busn_start=224, which is
>> the correct bus number base for this hardware.
>> 
>> Also add a PCI_POSSIBLE_ERROR() guard after reading VMCONFIG: a failed
>> config space read returns 0xFFFF, and BUS_RESTRICT_CFG(0xFFFF) = 3,
>> so without this guard a removed or errored device would falsely match
>> the new case 3 instead of being caught as an error.
> 
> I suppose this should fix
> https://bugzilla.kernel.org/show_bug.cgi?id=221136, which is a problem
> on an Arrow-Lake-S system?  I bcc'd the reporter, and we could add a
> Reported-by: and/or Closes: tag if appropriate.
> 
> That report mentions a 8086:09ab VMD device, and it looks like this
> patch addresses a 8086:ad0b device.  I suppose there are lots of VMD
> devices IDs that appear in lots of systems and implement this feature,
> which the bugzilla suggests is called "Dynamic Bus Offset".
> 
> Given that this is apparently a new VMD mode, AFAIK there is no public
> hardware spec for this, and the patch doesn't add any new code (and I
> would expect a new feature to require some new code), it would really
> be nice to have the Intel VMD maintainers chime in here.
> 
>> Signed-off-by: Ali Alaei <ali.alaei.tabatabaei@gmail.com>
>> ---
>> Changes in v3:
>> - Move changelog to after '---' (was incorrectly in commit message body)
>> - Add missing VMD maintainers to CC (Nirmal Patel, Jonathan Derrick)
>> 
>> Changes in v2:
>> - Add PCI_POSSIBLE_ERROR() guard after VMCONFIG read to prevent a
>>   failed config read (0xFFFF) from falsely matching case 3
>> 
>> drivers/pci/controller/vmd.c | 4 ++++
>> 1 file changed, 4 insertions(+)
>> 
>> diff --git a/drivers/pci/controller/vmd.c b/drivers/pci/controller/vmd.c
>> index xxxxxxx..yyyyyyy 100644
>> --- a/drivers/pci/controller/vmd.c
>> +++ b/drivers/pci/controller/vmd.c
>> @@ -640,6 +640,8 @@ static int vmd_get_bus_number_start(struct vmd_dev *vmd)
>> pci_read_config_word(dev, PCI_REG_VMCAP, &reg);
>> if (BUS_RESTRICT_CAP(reg)) {
>> pci_read_config_word(dev, PCI_REG_VMCONFIG, &reg);
>> + if (PCI_POSSIBLE_ERROR(reg))
>> + return -ENODEV;
>> 
>> switch (BUS_RESTRICT_CFG(reg)) {
>> case 0:
>> @@ -651,6 +653,7 @@ static int vmd_get_bus_number_start(struct vmd_dev *vmd)
>> case 1:
>> vmd->busn_start = 128;
>> break;
>> + case 3:
>> case 2:
>> vmd->busn_start = 224;
>> break;
>> --
>> 2.49.0
Hi Bjorn,

I am Lin Mohan, the reporter of
https://bugzilla.kernel.org/show_bug.cgi?id=221137.

Since commit 22b47bd49e45 ("PCI: vmd: Handle BUS_RESTRICT_CFG value 3
for Arrow Lake-HX") has already been merged, could you please add the
following tags in a follow-up fix?

Reported-by: Lin Mohan <linmhwork@outlook.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=221137
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=221136

This patch fixes the same Mode 3 / Dynamic Bus Offset issue I
reported on the 8086:09ab VMD device (Arrow Lake-S).

Thanks,
Lin Mohan