Re: [PATCH] PCI: Disable NoSnoop and Relaxed ordering for Intel wireless BE200

["Grumbach, Emmanuel" <emmanuel.grumbach@intel.com>]

On Fri, 2026-07-31 at 10:24 -0500, Bjorn Helgaas wrote:
> On Thu, Jul 30, 2026 at 08:16:33PM +0000, Grumbach, Emmanuel wrote:
> > On Thu, 2026-07-30 at 15:08 -0500, Bjorn Helgaas wrote:
> > ...
> 
> > > I wonder if the PCI core should disable RO and NoSnoop on Root
> > > Ports across the board?
> > > 
> > > I think this would mainly affect MMIO accesses performed by
> > > drivers, and I suspect drivers expect those writes to be
> > > performed
> > > in order, e.g., if the driver writes DMA descriptors to the
> > > device
> > > followed by a write to a doorbell, those should stay in order. 
> > > If
> > > the RP has RO enabled, those writes may be reordered.
> > 
> > As a device writer I'd say we'd be required to add a memory barrier
> > in that case, but we use writel usually.
> > Does that include a write vs write ordering? Hm...
> 
> Not being an ordering expert, I'm not sure I can make a convincing
> argument for whether RO should be enabled or disabled for Root Ports.
> 
> Maybe it's easier to argue that whatever the RO setting is, it should
> be the same across all platforms because drivers shouldn't have to
> adapt to platform differences.

That's playing safe, which is never a bad option if the performance hit
is not significant, but I can't tell I can claim that have any
expertise here.

> 
> Root Port settings potentially affect several endpoints, and I think
> it
> would be impractical for drivers to request RO settings that might
> conflict with each other.
> 
> Do you have any visibility into whether and how other OSes actively
> configure Root Port RO and NoSnoop across the board, independent of
> individual drivers?

I asked internally and we don't really know what Windows does when the
PCI root port has those bits sets. We mostly test with Intel / AMD
systems and those systems have those bits clear.

BTW - can you please point to the relevant paragraph in the PCI spec
that states RelaxedOrder and NoSnoop should be set by default on the
RP?