Merge branch 'pci/tph'

   pcieaer-howto

   endpoint/index

   boot-interrupts

   tph

.. SPDX-License-Identifier: GPL-2.0

===========

TPH Support

===========

:Copyright: 2024 Advanced Micro Devices, Inc.

:Authors: - Eric van Tassell <eric.vantassell@amd.com>

          - Wei Huang <wei.huang2@amd.com>

Overview

========

TPH (TLP Processing Hints) is a PCIe feature that allows endpoint devices

to provide optimization hints for requests that target memory space.

These hints, in a format called Steering Tags (STs), are embedded in the

requester's TLP headers, enabling the system hardware, such as the Root

Complex, to better manage platform resources for these requests.

For example, on platforms with TPH-based direct data cache injection

support, an endpoint device can include appropriate STs in its DMA

traffic to specify which cache the data should be written to. This allows

the CPU core to have a higher probability of getting data from cache,

potentially improving performance and reducing latency in data

processing.

How to Use TPH

==============

TPH is presented as an optional extended capability in PCIe. The Linux

kernel handles TPH discovery during boot, but it is up to the device

driver to request TPH enablement if it is to be utilized. Once enabled,

the driver uses the provided API to obtain the Steering Tag for the

target memory and to program the ST into the device's ST table.

Enable TPH support in Linux

---------------------------

To support TPH, the kernel must be built with the CONFIG_PCIE_TPH option

enabled.

Manage TPH

----------

To enable TPH for a device, use the following function::

  int pcie_enable_tph(struct pci_dev *pdev, int mode);

This function enables TPH support for device with a specific ST mode.

Current supported modes include:

  * PCI_TPH_ST_NS_MODE - NO ST Mode

  * PCI_TPH_ST_IV_MODE - Interrupt Vector Mode

  * PCI_TPH_ST_DS_MODE - Device Specific Mode

`pcie_enable_tph()` checks whether the requested mode is actually

supported by the device before enabling. The device driver can figure out

which TPH mode is supported and can be properly enabled based on the

return value of `pcie_enable_tph()`.

To disable TPH, use the following function::

  void pcie_disable_tph(struct pci_dev *pdev);

Manage ST

---------

Steering Tags are platform specific. PCIe spec does not specify where STs

are from. Instead PCI Firmware Specification defines an ACPI _DSM method

(see the `Revised _DSM for Cache Locality TPH Features ECN

<https://members.pcisig.com/wg/PCI-SIG/document/15470>`_) for retrieving

STs for a target memory of various properties. This method is what is

supported in this implementation.

To retrieve a Steering Tag for a target memory associated with a specific

CPU, use the following function::

  int pcie_tph_get_cpu_st(struct pci_dev *pdev, enum tph_mem_type type,

                          unsigned int cpu_uid, u16 *tag);

The `type` argument is used to specify the memory type, either volatile

or persistent, of the target memory. The `cpu_uid` argument specifies the

CPU where the memory is associated to.

After the ST value is retrieved, the device driver can use the following

function to write the ST into the device::

  int pcie_tph_set_st_entry(struct pci_dev *pdev, unsigned int index,

                            u16 tag);

The `index` argument is the ST table entry index the ST tag will be

written into. `pcie_tph_set_st_entry()` will figure out the proper

location of ST table, either in the MSI-X table or in the TPH Extended

Capability space, and write the Steering Tag into the ST entry pointed by

the `index` argument.

It is completely up to the driver to decide how to use these TPH

functions. For example a network device driver can use the TPH APIs above

to update the Steering Tag when interrupt affinity of a RX/TX queue has

been changed. Here is a sample code for IRQ affinity notifier:

.. code-block:: c

    static void irq_affinity_notified(struct irq_affinity_notify *notify,

                                      const cpumask_t *mask)

    {

         struct drv_irq *irq;

         unsigned int cpu_id;

         u16 tag;

         irq = container_of(notify, struct drv_irq, affinity_notify);

         cpumask_copy(irq->cpu_mask, mask);

         /* Pick a right CPU as the target - here is just an example */

         cpu_id = cpumask_first(irq->cpu_mask);

         if (pcie_tph_get_cpu_st(irq->pdev, TPH_MEM_TYPE_VM, cpu_id,

                                 &tag))

             return;

         if (pcie_tph_set_st_entry(irq->pdev, irq->msix_nr, tag))

             return;

    }

Disable TPH system-wide

-----------------------

There is a kernel command line option available to control TPH feature:

    * "notph": TPH will be disabled for all endpoint devices.

		nomio		[S390] Do not use MIO instructions.

		norid		[S390] ignore the RID field and force use of

				one PCI domain per PCI function

		notph		[PCIE] If the PCIE_TPH kernel config parameter

				is enabled, this kernel boot option can be used

				to disable PCIe TLP Processing Hints support

				system-wide.

	pcie_aspm=	[PCIE] Forcibly enable or ignore PCIe Active State Power

			Management.

.. kernel-doc:: drivers/pci/pci-sysfs.c

   :internal:

.. kernel-doc:: drivers/pci/tph.c

   :export:

PCI Hotplug Support Library

---------------------------

	  If unsure, say N.

config PCIE_TPH

	bool "TLP Processing Hints"

	help

	  This option adds support for PCIe TLP Processing Hints (TPH).

	  TPH allows endpoint devices to provide optimization hints, such as

	  desired caching behavior, for requests that target memory space.

	  These hints, called Steering Tags, can empower the system hardware

	  to optimize the utilization of platform resources.

config PCI_P2PDMA

	bool "PCI peer-to-peer transfer support"

	depends on ZONE_DEVICE