Re: [RFC PATCH 1/1] PCI: Add Extended Tag + MRRS quirk for Xeon 6

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Mar 07, 2025 at 03:06:31PM +0200, Ilpo Järvinen wrote:
> On Tue, 4 Mar 2025, Bjorn Helgaas wrote:
> > On Tue, Mar 04, 2025 at 03:51:08PM +0200, Ilpo Järvinen wrote:
> > > Disallow Extended Tags and Max Read Request Size (MRRS) larger than
> > > 128B for devices under Xeon 6 Root Ports if the Root Port is bifurcated
> > > to x2. Also, 10-Bit Tag Requester should be disallowed for device
> > > underneath these Root Ports but there is currently no 10-Bit Tag
> > > support in the kernel.
> > > 
> > > The normal path that writes MRRS is through
> > > pcie_bus_configure_settings() -> pcie_bus_configure_set() ->
> > > pcie_write_mrrs() and contains a few early returns that are based on
> > > the value of pcie_bus_config. Overriding such checks with the host
> > > bridge flag check on each level seems messy. Thus, simply ensure MRRS
> > > is always written in pci_configure_device() if a device requiring the
> > > quirk is detected.
> > 
> > This is kind of weird.  It's apparently not an erratum in the sense
> > that something doesn't *work*, just something for "optimized PCIe
> > performance"?
> > 
> > What are we supposed to do with this?  Add similar quirks for every
> > random PCI controller?  Scratching my head about what this means for
> > the future.
> > 
> > What bad things happen if we *don't* do this?  Is this something we
> > could/should rely on BIOS to configure for us?
> 
> Even if BIOS configures this (I'm under impression they already do, I 
> had problem in finding a configuration in our lab on which this patch
> had some effect). But my kernel was built with CONFIG_PCIE_BUS_DEFAULT, if 
> I set that to CONFIG_PCIE_BUS_PERFORMANCE, what BIOS did will be 
> overwritten.

I despise those CONFIG_PCIE_BUS_* options, but have never managed to
get rid of them.  Unfortunate that something named "*_PERFORMANCE"
will apparently result in *worse* performance in this respect.