Re: [RFC 2/9] cxl: Add preserve_config to pxb-cxl OSC method

[Jonathan Cameron <jic23@kernel.org>]

On Mon, 27 Apr 2026 23:42:28 +0530
<mhonap@nvidia.com> wrote:

> From: Manish Honap <mhonap@nvidia.com>
> 
> Before this patch, pxb-cxl bridges had no _DSM method at all. When the
> OS called _DSM on a CXL host bridge, ACPI returned an error and the OS
> defaulted to reassigning resources across suspend/resume. On machines
> where firmware pre-commits the HDM decoder, that reassignment breaks the
> DPA mapping.

Ah. Ok. So it's suspend / resume that is causing trouble. I wasn't aware
PCI reenumerated on that as opposed to boot. I surprised more doesn't
break.  Make sure to call that out in the cover letter as the impression
I got was the problem was initial enumeration.

Mind you I'm still confused. Why is CXL.mem path affected?  That's
nothing to do with PCI enumeration.

Jonathan

> 
> Wire preserve_config through GPEXConfig into build_cxl_osc_method() so
> pxb-cxl host bridges get a _DSM method that signals the OS to keep
> resource assignments stable when needed. The _DSM function 5 (preserve
> firmware PCI configuration) is the mechanism used to convey this.
> 
> build_pci_host_bridge_dsm_method() is promoted from static to exported
> so cxl.c can call it without duplicating the AML.
> 
> The x86 build_cxl_osc_method() call site passes false since x86 does
> not use firmware-committed HDM decoders.
> 
> build_cxl_osc_method is renamed to acpi_dsdt_add_cxl_host_bridge_methods
> The function now appends both the CXL _OSC method and the _DSM method,
> so its old name is misleading. Renamed it to match the pxb-pcie analogue
> acpi_dsdt_add_host_bridge_methods(), making the two root bridge code
> paths symmetric. No AML change.
> 
> Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
> Signed-off-by: Manish Honap <mhonap@nvidia.com>
> ---
>  hw/acpi/cxl-stub.c         | 2 +-
>  hw/acpi/cxl.c              | 4 +++-
>  hw/i386/acpi-build.c       | 2 +-
>  hw/pci-host/gpex-acpi.c    | 5 +++--
>  include/hw/acpi/cxl.h      | 2 +-
>  include/hw/pci-host/gpex.h | 1 +
>  6 files changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/hw/acpi/cxl-stub.c b/hw/acpi/cxl-stub.c
> index 15bc21076b..d7c6731975 100644
> --- a/hw/acpi/cxl-stub.c
> +++ b/hw/acpi/cxl-stub.c
> @@ -6,7 +6,7 @@
>  #include "hw/acpi/aml-build.h"
>  #include "hw/acpi/cxl.h"
>  
> -void build_cxl_osc_method(Aml *dev)
> +void acpi_dsdt_add_cxl_host_bridge_methods(Aml *dev, bool preserve_config)
>  {
>      g_assert_not_reached();
>  }
> diff --git a/hw/acpi/cxl.c b/hw/acpi/cxl.c
> index f92f7fa3d5..b32740a3e3 100644
> --- a/hw/acpi/cxl.c
> +++ b/hw/acpi/cxl.c
> @@ -23,6 +23,7 @@
>  #include "hw/pci/pci_host.h"
>  #include "hw/cxl/cxl.h"
>  #include "hw/cxl/cxl_host.h"
> +#include "hw/pci-host/gpex.h"
>  #include "hw/mem/memory-device.h"
>  #include "hw/acpi/acpi.h"
>  #include "hw/acpi/aml-build.h"
> @@ -320,11 +321,12 @@ static Aml *__build_cxl_osc_method(void)
>      return method;
>  }
>  
> -void build_cxl_osc_method(Aml *dev)
> +void acpi_dsdt_add_cxl_host_bridge_methods(Aml *dev, bool preserve_config)
>  {
>      aml_append(dev, aml_name_decl("SUPP", aml_int(0)));
>      aml_append(dev, aml_name_decl("CTRL", aml_int(0)));
>      aml_append(dev, aml_name_decl("SUPC", aml_int(0)));
>      aml_append(dev, aml_name_decl("CTRC", aml_int(0)));
>      aml_append(dev, __build_cxl_osc_method());
> +    aml_append(dev, build_pci_host_bridge_dsm_method(preserve_config));
>  }
> diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> index f622b91b76..f66ec8ed24 100644
> --- a/hw/i386/acpi-build.c
> +++ b/hw/i386/acpi-build.c
> @@ -1013,7 +1013,7 @@ build_dsdt(GArray *table_data, BIOSLinker *linker,
>                  aml_append(aml_pkg, aml_eisaid("PNP0A08"));
>                  aml_append(aml_pkg, aml_eisaid("PNP0A03"));
>                  aml_append(dev, aml_name_decl("_CID", aml_pkg));
> -                build_cxl_osc_method(dev);
> +                acpi_dsdt_add_cxl_host_bridge_methods(dev, false);
>              } else if (pci_bus_is_express(bus)) {
>                  aml_append(dev, aml_name_decl("_HID", aml_eisaid("PNP0A08")));
>                  aml_append(dev, aml_name_decl("_CID", aml_eisaid("PNP0A03")));
> diff --git a/hw/pci-host/gpex-acpi.c b/hw/pci-host/gpex-acpi.c
> index 7de57bbc46..247bd78152 100644
> --- a/hw/pci-host/gpex-acpi.c
> +++ b/hw/pci-host/gpex-acpi.c
> @@ -52,7 +52,7 @@ static void acpi_dsdt_add_pci_route_table(Aml *dev, uint32_t irq,
>      }
>  }
>  
> -static Aml *build_pci_host_bridge_dsm_method(bool preserve_config)
> +Aml *build_pci_host_bridge_dsm_method(bool preserve_config)
>  {
>      Aml *method = aml_method("_DSM", 4, AML_NOTSERIALIZED);
>      Aml *UUID, *ifctx, *ifctx1, *buf;
> @@ -204,7 +204,8 @@ void acpi_dsdt_add_gpex(Aml *scope, struct GPEXConfig *cfg)
>              aml_append(dev, aml_name_decl("_CRS", crs));
>  
>              if (is_cxl) {
> -                build_cxl_osc_method(dev);
> +                acpi_dsdt_add_cxl_host_bridge_methods(dev,
> +                                                      cfg->preserve_config);
>              } else {
>                  /* pxb bridges do not have ACPI PCI Hot-plug enabled */
>                  acpi_dsdt_add_host_bridge_methods(dev, true,
> diff --git a/include/hw/acpi/cxl.h b/include/hw/acpi/cxl.h
> index 8f22c71530..6fe6c9c58d 100644
> --- a/include/hw/acpi/cxl.h
> +++ b/include/hw/acpi/cxl.h
> @@ -24,7 +24,7 @@
>  void cxl_build_cedt(GArray *table_offsets, GArray *table_data,
>                      BIOSLinker *linker, const char *oem_id,
>                      const char *oem_table_id, CXLState *cxl_state);
> -void build_cxl_osc_method(Aml *dev);
> +void acpi_dsdt_add_cxl_host_bridge_methods(Aml *dev, bool preserve_config);
>  void build_cxl_dsm_method(Aml *dev);
>  
>  #endif
> diff --git a/include/hw/pci-host/gpex.h b/include/hw/pci-host/gpex.h
> index a7c2e2edf3..e5c2ebef78 100644
> --- a/include/hw/pci-host/gpex.h
> +++ b/include/hw/pci-host/gpex.h
> @@ -73,6 +73,7 @@ struct GPEXHost {
>  int gpex_set_irq_num(GPEXHost *s, int index, int gsi);
>  
>  void acpi_dsdt_add_gpex(Aml *scope, struct GPEXConfig *cfg);
> +Aml *build_pci_host_bridge_dsm_method(bool preserve_config);
>  void acpi_dsdt_add_gpex_host(Aml *scope, uint32_t irq);
>  
>  #define PCI_HOST_PIO_BASE               "x-pio-base"