Re: RFC about how to obtain PCIE TPH steer-tag on ARM64 platform

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Jan 12, 2026 at 11:01:31AM +0800, fengchengwen wrote:
> Hi all,
> 
> We want to enable PCIE TPH feature on ARM64 platform, but we encounter the
> following problem:
>
> 1. The pcie_tph_get_cpu_st() function invokes the ACPI DSM method to
>    obtain the steer-tag of the CPU. According to the definition of
>    the DSM method [1], the cpu_uid should be "ACPI processor uid".

> 2. In the current implementation, the ACPI DSM method is invoked
>    directly using the logical core number, which works on the x86
>    platform but does not work on the ARM64 platform because the
>    logical core ID is not the same as the ACPI processor ID when the
>    PG exists.

PG?

> Because the ARM64 platform generates steer-tag based on the MPIDR
> information (at least for the Kunpeng platform). Therefore, we have
> two option:
>
> Option-1: convert logic core ID to ACPI process ID: use
>           get_acpi_id_for_cpu() to get ACPI process ID in
>           pcie_tph_get_cpu_st() before invoke dsm [2], and BIOS/ACPI
>           use process ID to get corresponding MPIDR, and then
>           generate steer-tag from MPIDR.
>
> Option-2: convert logic core ID to MPIDR: use cpu_logical_map() to
>           get MPIDR in pcie_tph_get_cpu_st() before invoke dsm, and
>           BIOS/ACPI use it to generate steer-tag directly.
> 
> Option-1 complies with _DSM ECN, but requires BIOS/ACPI to maintain
> a mapping table from acpi_process_id to MPIDR.
>
> Option 2 does not comply with _DSM ECN (if extension is required).
> But it is easy to implement and can be extended to the DT system
> (ACPI is not supported) I think.

Sounds like this would be of interest to any OS, not just Linux.

Possibly a topic for the PCI-SIG firmware working group
(https://members.pcisig.com/wg/Firmware/dashboard) or the ACPI spec
working group (https://uefi.org/workinggroups)?

> [1] According to _DSM ECN, the input is defined as: "If the target
>     is a processor, then this field represents the ACPI Processor
>     UID of the processor as specified in the MADT. If the target is
>     a processor container, then this field represents the ACPI
>     Processor UID of the processor container as specified in the
>     PPTT"
>
> [2] git diff about /drivers/pci/tph.c
> @@ -289,6 +289,9 @@ int pcie_tph_get_cpu_st(struct pci_dev *pdev, enum tph_mem_type mem_type,
> 
>         rp_acpi_handle = ACPI_HANDLE(rp->bus->bridge);
> 
> +#ifdef CONFIG_ARM64
> +       cpu_uid = get_acpi_id_for_cpu(cpu_uid);
> +#endif
>         if (tph_invoke_dsm(rp_acpi_handle, cpu_uid, &info) != AE_OK) {
>                 *tag = 0;
>                 return -EINVAL;
> 
>