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

[fengchengwen <fengchengwen@huawei.com>]

On 1/14/2026 3:07 AM, Bjorn Helgaas wrote:
> 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?

partial good

> 
>> 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)?

Thanks
We finally decided to choose option-1.

> 
>> [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;
>>
>>
>