11. TPH Support

Copyright:

2024 Advanced Micro Devices, Inc.

Authors:

11.1. 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.

11.2. 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.

11.2.1. Enable TPH support in Linux

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

11.2.2. 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);

11.2.3. 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) 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:

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;
}

11.2.4. 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.