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

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

[fengchengwen]

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.


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.


Hope for your feedback.

With best regards,
Chengwen


[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;

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

[Bjorn Helgaas]

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;
> 
> 

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

[Wei Huang]

On Tue, Jan 13, 2026 at 01:07:13PM -0600, 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?
> 
> > 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.

In my opinion, if this is achievable in your BIOS/ACPI, it is clean vs. Option-2 and preferred.

> >
> > 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.

This solution requires you to change the ECN and ratify it (as suggested by Bjorn below). The implementation can also be complicated.

> > 
> > 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.

If you plan to revamp it, one (wild) idea is that ST retrieval can be extended to support:
  1. ACPI _DSM
  2. DT
  3. Vendor specific

After that, your MPIDR solution can be plugged-in under (3).

> 
> 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;
> > 
> > 

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

[fengchengwen]

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;
>>
>>
> 

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

[fengchengwen]

On 1/14/2026 6:38 AM, Wei Huang wrote:
> On Tue, Jan 13, 2026 at 01:07:13PM -0600, 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?
>>
>>> 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.
> 
> In my opinion, if this is achievable in your BIOS/ACPI, it is clean vs. Option-2 and preferred.

Yes, it's achievable for out BIOS/ACPI, and we will adopt this option.

> 
>>>
>>> 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.
> 
> This solution requires you to change the ECN and ratify it (as suggested by Bjorn below). The implementation can also be complicated.

Indeed, and it may take a long time.

@Wei, I have another question:
The cache is hierarchical, for example, L2 or L3. Does the DSM mode support the specified cache level?
I checked the ECN doc and there are "Cache Reference" fields, but the kernel code don't enable them.

> 
>>>
>>> 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.
> 
> If you plan to revamp it, one (wild) idea is that ST retrieval can be extended to support:
>   1. ACPI _DSM
>   2. DT
>   3. Vendor specific
> 
> After that, your MPIDR solution can be plugged-in under (3).
> 
>>
>> 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;
>>>
>>>
> 

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

[Wei Huang]

On Wed, Jan 14, 2026 at 11:52:50AM +0800, fengchengwen wrote:
> On 1/14/2026 6:38 AM, Wei Huang wrote:
> > On Tue, Jan 13, 2026 at 01:07:13PM -0600, 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?
> >>
> >>> 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.
> > 
> > In my opinion, if this is achievable in your BIOS/ACPI, it is clean vs. Option-2 and preferred.
> 
> Yes, it's achievable for out BIOS/ACPI, and we will adopt this option.
> 
> > 
> >>>
> >>> 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.
> > 
> > This solution requires you to change the ECN and ratify it (as suggested by Bjorn below). The implementation can also be complicated.
> 
> Indeed, and it may take a long time.
> 
> @Wei, I have another question:
> The cache is hierarchical, for example, L2 or L3. Does the DSM mode support the specified cache level?
> I checked the ECN doc and there are "Cache Reference" fields, but the kernel code don't enable them.

ECN does provide the "Cache Reference" field as part of argument2. Our platforms don't use it and don't plan to use it soon. You can explore the use cases if your platforms need it.

I want to point out that PCIe TLP header doesn't have a field for cache level. So no matter what, TLP header will stuck with 8- or 16-bit STs.

> 

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

[Bjorn Helgaas]

On Wed, Jan 14, 2026 at 11:39:27AM +0800, fengchengwen wrote:
> 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

I still don't know what "partial good" means :)  Is that something
from a spec?

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

[fengchengwen]

On 1/15/2026 12:36 AM, Wei Huang wrote:
> On Wed, Jan 14, 2026 at 11:52:50AM +0800, fengchengwen wrote:
>> On 1/14/2026 6:38 AM, Wei Huang wrote:
>>> On Tue, Jan 13, 2026 at 01:07:13PM -0600, 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?
>>>>
>>>>> 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.
>>>
>>> In my opinion, if this is achievable in your BIOS/ACPI, it is clean vs. Option-2 and preferred.
>>
>> Yes, it's achievable for out BIOS/ACPI, and we will adopt this option.
>>
>>>
>>>>>
>>>>> 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.
>>>
>>> This solution requires you to change the ECN and ratify it (as suggested by Bjorn below). The implementation can also be complicated.
>>
>> Indeed, and it may take a long time.
>>
>> @Wei, I have another question:
>> The cache is hierarchical, for example, L2 or L3. Does the DSM mode support the specified cache level?
>> I checked the ECN doc and there are "Cache Reference" fields, but the kernel code don't enable them.
> 
> ECN does provide the "Cache Reference" field as part of argument2. Our platforms don't use it and don't plan to use it soon. You can explore the use cases if your platforms need it.
> 
> I want to point out that PCIe TLP header doesn't have a field for cache level. So no matter what, TLP header will stuck with 8- or 16-bit STs.

Okay, I will try to understand how to use "Cache Reference" as cache level parameter.
I think this would give the user a more refined ability.

> 
>>
> 
> 

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

[fengchengwen]

On 1/15/2026 1:03 AM, Bjorn Helgaas wrote:
> On Wed, Jan 14, 2026 at 11:39:27AM +0800, fengchengwen wrote:
>> 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
> 
> I still don't know what "partial good" means :)  Is that something
> from a spec?

Because of some issues (like manufacturing variances), certain circuits
(e.g., some cores) might not work normally. For these cases, we can use
the part at a reduced specification, which is the PG mentioned here.

> 

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

[Bjorn Helgaas]

On Fri, Jan 16, 2026 at 08:21:30AM +0800, fengchengwen wrote:
> On 1/15/2026 1:03 AM, Bjorn Helgaas wrote:
> > On Wed, Jan 14, 2026 at 11:39:27AM +0800, fengchengwen wrote:
> >> 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
> > 
> > I still don't know what "partial good" means :)  Is that something
> > from a spec?
> 
> Because of some issues (like manufacturing variances), certain circuits
> (e.g., some cores) might not work normally. For these cases, we can use
> the part at a reduced specification, which is the PG mentioned here.

Ah, got it :)  If you go this direction, maybe you can expand that
part of the commit log a bit, since I don't think "PG" or "partial
good" is a widely-known term.

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

[fengchengwen]

On 1/17/2026 3:32 AM, Bjorn Helgaas wrote:
> On Fri, Jan 16, 2026 at 08:21:30AM +0800, fengchengwen wrote:
>> On 1/15/2026 1:03 AM, Bjorn Helgaas wrote:
>>> On Wed, Jan 14, 2026 at 11:39:27AM +0800, fengchengwen wrote:
>>>> 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
>>>
>>> I still don't know what "partial good" means :)  Is that something
>>> from a spec?
>>
>> Because of some issues (like manufacturing variances), certain circuits
>> (e.g., some cores) might not work normally. For these cases, we can use
>> the part at a reduced specification, which is the PG mentioned here.
> 
> Ah, got it :)  If you go this direction, maybe you can expand that
> part of the commit log a bit, since I don't think "PG" or "partial
> good" is a widely-known term.

Alright, thank you for the reminder.

>