16-bit _SEG vs 8 bit PCIe Flit mode Segment

16-bit _SEG vs 8 bit PCIe Flit mode Segment

[Bjorn Helgaas]

ACPI r6.5, sec 6.5.6, currently says the low 16 bits of _SEG are the
PCI Segment Group number.  PCIe r6.0, sec 2.2.1.2, added Flit mode
with TLP headers that may contain an 8-bit Segment number.

ACPI currently says _SEG is purely a software thing and has no
connection to any physical entities.  But this may get a little blurry
when Segment numbers appear in TLPs.  For example, AER header logs
will likely contain the Flit Segment, and we'll need to correlate that
with the _SEG-derived identifiers Linux uses.

One possibility is to reduce the width of _SEG to 8 bits to match the
Flit mode Segment and require them to be identical.

I'm trying to figure out whether that would break any existing
systems.  I've heard rumors that large systems like SGI UV may use
more than 8 bits of _SEG.  But I don't know any details.

Bjorn

Re: 16-bit _SEG vs 8 bit PCIe Flit mode Segment

[Dimitri Sivanich]

Bjorn,

Thanks for letting us know about this.

This is just to let you know that, while we are still investigating this,
our UV3 Broadwell era systems do use more than 8 bits for the segment #.

From lscpi on UV3:
  1007:3f:08.0 System peripheral: Intel Corporation Xeon E7 v2/Xeon E5 v2/Core i7 QPI Link 0 (rev 07)

Dimitri

On Tue, Nov 01, 2022 at 09:49:05AM -0500, Bjorn Helgaas wrote:
> ACPI r6.5, sec 6.5.6, currently says the low 16 bits of _SEG are the
> PCI Segment Group number.  PCIe r6.0, sec 2.2.1.2, added Flit mode
> with TLP headers that may contain an 8-bit Segment number.
> 
> ACPI currently says _SEG is purely a software thing and has no
> connection to any physical entities.  But this may get a little blurry
> when Segment numbers appear in TLPs.  For example, AER header logs
> will likely contain the Flit Segment, and we'll need to correlate that
> with the _SEG-derived identifiers Linux uses.
> 
> One possibility is to reduce the width of _SEG to 8 bits to match the
> Flit mode Segment and require them to be identical.
> 
> I'm trying to figure out whether that would break any existing
> systems.  I've heard rumors that large systems like SGI UV may use
> more than 8 bits of _SEG.  But I don't know any details.
> 
> Bjorn