Re: [PATCH] Documentation: PCI: add vmd documentation

[Paul M Stillwell Jr <paul.m.stillwell.jr@intel.com>]

On 4/23/2024 2:26 PM, Bjorn Helgaas wrote:
> On Mon, Apr 22, 2024 at 04:39:19PM -0700, Paul M Stillwell Jr wrote:
>> On 4/22/2024 3:52 PM, Bjorn Helgaas wrote:
>>> On Mon, Apr 22, 2024 at 02:39:16PM -0700, Paul M Stillwell Jr wrote:
>>>> On 4/22/2024 1:27 PM, Bjorn Helgaas wrote:
>> ...
> 
>>>>> _OSC negotiates ownership of features between platform firmware and
>>>>> OSPM.  The "native_pcie_hotplug" and similar bits mean that "IF a
>>>>> device advertises the feature, the OS can use it."  We clear those
>>>>> native_* bits if the platform retains ownership via _OSC.
>>>>>
>>>>> If BIOS doesn't enable the VMD host bridge and doesn't supply _OSC for
>>>>> the domain below it, why would we assume that BIOS retains ownership
>>>>> of the features negotiated by _OSC?  I think we have to assume the OS
>>>>> owns them, which is what happened before 04b12ef163d1.
>>>>
>>>> Sorry, this confuses me :) If BIOS doesn't enable VMD (i.e. VMD is disabled)
>>>> then all the root ports and devices underneath VMD are visible to the BIOS
>>>> and OS so ACPI would run on all of them and the _OSC bits should be set
>>>> correctly.
>>>
>>> Sorry, that was confusing.  I think there are two pieces to enabling
>>> VMD:
>>>
>>>     1) There's the BIOS "VMD enable" switch.  If set, the VMD device
>>>     appears as an RCiEP and the devices behind it are invisible to the
>>>     BIOS.  If cleared, VMD doesn't exist; the VMD RCiEP is hidden and
>>>     the devices behind it appear as normal Root Ports with devices below
>>>     them.
>>>
>>>     2) When the BIOS "VMD enable" is set, the OS vmd driver configures
>>>     the VMD RCiEP and enumerates things below the VMD host bridge.
>>>
>>>     In this case, BIOS enables the VMD RCiEP, but it doesn't have a
>>>     driver for it and it doesn't know how to enumerate the VMD Root
>>>     Ports, so I don't think it makes sense for BIOS to own features for
>>>     devices it doesn't know about.
>>
>> That makes sense to me. It sounds like VMD should own all the features, I
>> just don't know how the vmd driver would set the bits other than hotplug
>> correctly... We know leaving them on is problematic, but I'm not sure what
>> method to use to decide which of the other bits should be set or not.
> 
> My starting assumption would be that we'd handle the VMD domain the
> same as other PCI domains: if a device advertises a feature, the
> kernel includes support for it, and the kernel owns it, we enable it.
> 

I've been poking around and it seems like some things (I was looking for 
AER) are global to the platform. In my investigation (which is a small 
sample size of machines) it looks like there is a single entry in the 
BIOS to enable/disable AER so whatever is in one domain should be the 
same in all the domains. I couldn't find settings for LTR or the other 
bits, but I'm not sure what to look for in the BIOS for those.

So it seems that there are 2 categories: platform global and device 
specific. AER and probably some of the others are global and can be 
copied from one domain to another, but things like hotplug are device 
specific and should be handled that way.

Based on this it seems like my patch here 
https://lore.kernel.org/linux-pci/20240408183927.135-1-paul.m.stillwell.jr@intel.com/ 
is probably the correct thing to do, but I think adding some output into 
dmesg to indicate that VMD owns hotplug and has enabled it needs to be done.

> If a device advertises a feature but there's a hardware problem with
> it, the usual approach is to add a quirk to work around the problem.
> The Correctable Error issue addressed by 04b12ef163d1 ("PCI: vmd:
> Honor ACPI _OSC on PCIe features"), looks like it might be in this
> category.
> 

I don't think we had a hardware problem with these Samsung (IIRC) 
devices; the issue was that the vmd driver were incorrectly enabling AER 
because those native_* bits get set automatically. I think 04b12ef163d1 
is doing the correct thing, but it is incorrectly copying the hotplug 
bits. Those are device specific and should be handled by the device 
instead of based on another domain.

Paul