Re: [PATCH 1/1] cxl/acpi.c: Add buggy BIOS hint for CXL ACPI lookup failure

[PJ Waskiewicz <ppwaskie@kernel.org>]

On Wed, 2024-05-01 at 12:54 -0500, Bjorn Helgaas wrote:
> On Wed, May 01, 2024 at 08:28:22AM -0700, PJ Waskiewicz wrote:
> > On Mon, 2024-04-29 at 11:35 -0700, Dan Williams wrote:
> > > Bjorn Helgaas wrote:
> > > > On Sun, Apr 28, 2024 at 10:57:13PM -0700, PJ Waskiewicz wrote:
> > > > > On Tue, 2024-04-09 at 08:22 -0500, Bjorn Helgaas wrote:
> > > > > > On Sun, Apr 07, 2024 at 02:05:26PM -0700,
> > > > > > ppwaskie@kernel.org wrote:
> > > > > > > From: PJ Waskiewicz <ppwaskie@kernel.org>
> > > > > > > 
> > > > > > > Currently, Type 3 CXL devices (CXL.mem) can train using
> > > > > > > host CXL drivers on Emerald Rapids systems.  However, on
> > > > > > > some production systems from some vendors, a buggy BIOS
> > > > > > > exists that improperly populates the ACPI => PCI
> > > > > > > mappings.
> > > > > > 
> > > > > > Can you be more specific about what this ACPI => PCI
> > > > > > mapping
> > > > > > is?  If you already know what the problem is, I'm sure this
> > > > > > is obvious, but otherwise it's not.
> > > [..] 
> > > > It's just a buggy BIOS that doesn't supply _UID for an ACPI0016
> > > > object, so you can't locate the corresponding CEDT entry,
> > > > right?
> > > 
> > > Correct, the problem is 100% contained to ACPI, and PCI is
> > > innocent.  The ACPI bug leads to failures to associate ACPI
> > > host-bridge objects with CEDT.CHBS entries.
> > 
> > Sorry for the confusion here!!  I was definitely not trying to
> > blame
> > PCI.  :)
> > 
> > > ACPI to PCI association is then typical pci_root lookup, i.e.:
> > > 
> > >         pci_root = acpi_pci_find_root(hb->handle);
> > >         bridge = pci_root->bus->bridge;
> > 
> > Yes, this here.  In my use case, I'm starting with a PCIe/CXL
> > device.
> > In my driver, I try to discover the host bridge, and then the ACPI
> > _UID
> > so I can look things up in the CEDT.
> > 
> > So I'm trying to do the programmatic equivalent of this:
> > 
> > Start here in my PCIe/CXL host driver:
> > 
> > /sys/devices/pci0000:37/firmware_node =>
> > ../LNXSYSTM:00/LNXSYBUS:00/ACPI0016:02
> > 
> > Retrieve _UID (uid) from /sys/devices/pci0000:37/firmware_node/uid
> > 
> > Buggy BIOS, that above value resolves to CX02.  In fact, it
> > *should* be
> > 49.  This is very much a bug in the ACPI arena.
> > 
> > The kernel APIs allowing me to walk this path would fail in the
> > acpi_evaluate_object() when trying to pass in the bad _UID (CX02).
> > 
> > Again, sorry for the confusion if it looked like I was trying to
> > implicate PCI in any way.  The whole intent here was to leave some
> > breadcrumbs so anyone else running into this wouldn't be left
> > scratching their heads wondering wtf was going on.
> 
> 
> No worries, I didn't suspect a PCI issue here; I just wasn't clear on
> what ACPI=>PCI mapping was involved.  It sounds like there *is* no
> such mapping in this picture (you find the ACPI object for a PCIe/CXL
> host bridge, evaluate _UID from that object, and get a bogus value).
> 
> So the commit log text:
> 
>   However, on some production systems from some vendors, a buggy BIOS
>   exists that improperly populates the ACPI => PCI mappings.
> 
> apparently refers to improper implementation of the _UID, which
> doesn't return anything PCI related.

Agreed.  I'm happy to fix the commit message to be more accurate, if we
move forward with rolling this or Dan's (better) approach to handling
this.

> 
> It also says:
> 
>   This leads to the cxl_acpi driver to fail probe when it cannot find
>   the root port's _UID, in order to look up the device's CXL
>   attributes in the CEDT.
> 
> I *think* strictly speaking this should refer to the *host bridge's*
> _UID, not the Root Port's, e.g., something like this:
> 
>   However, on some production systems from some vendors, a buggy BIOS
>   provides a CXL host bridge _UID that doesn't match anything in the
>   CEDT.

Much better description.  I'll roll it in.

I appreciate the look-over and inputs!

-PJ