PCI/TPH: Add TPH documentation

Add a document for the TPH feature, including description of "notph" kernel
parameter and the API interface.

Co-developed-by: Eric Van Tassell <Eric.VanTassell@amd.com>
Link: https://lore.kernel.org/r/20241002165954.128085-4-wei.huang2@amd.com
Signed-off-by: Eric Van Tassell <Eric.VanTassell@amd.com>
Signed-off-by: Wei Huang <wei.huang2@amd.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>

Documentation/PCI/index.rst

@@ -18,3 +18,4 @@ PCI Bus Subsystem
    pcieaer-howto
    endpoint/index
    boot-interrupts
+   tph

Documentation/PCI/tph.rst

@@ -0,0 +1,132 @@
+.. 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.

Documentation/driver-api/pci/pci.rst

@@ -46,6 +46,9 @@ PCI Support Library
 .. kernel-doc:: drivers/pci/pci-sysfs.c
    :internal:
 
+.. kernel-doc:: drivers/pci/tph.c
+   :export:
+
 PCI Hotplug Support Library
 ---------------------------