[PATCH 1/2] switch_discovery: Add new module to discover inter switch links between PCI-to-PCI bridges

From: Shivasharan S
Date: Wed Jun 12 2024 - 07:38:39 EST

This kernel module discovers the virtual inter-switch P2P links present
between two PCI-to-PCI bridges that allows an optimal data path for data
movement. The module creates sysfs entries for upstream PCI-to-PCI
bridges which supports the inter switch P2P links as below:

Host root bridge
---------------------------------------
| |
NIC1 --- PCI Switch1 --- Inter-switch link --- PCI Switch2 --- NIC2
(af:00.0) (ad:00.0) (8b:00.0) (8d:00.0)
| |
GPU1 GPU2
(b0:00.0) (8e:00.0)
SERVER 1

/sys/kernel/pci_switch_link/virtual_switch_links
├── 0000:8b:00.0
│   └── 0000:ad:00.0 -> ../0000:ad:00.0
└── 0000:ad:00.0
└── 0000:8b:00.0 -> ../0000:8b:00.0

Signed-off-by: Shivasharan S <shivasharan.srikanteshwara@xxxxxxxxxxxx>
Signed-off-by: Sumanesh Samanta <sumanesh.samanta@xxxxxxxxxxxx>
---
.../driver-api/pci/switch_discovery.rst | 52 +++
MAINTAINERS | 13 +
drivers/pci/switch/Kconfig | 9 +
drivers/pci/switch/Makefile | 1 +
drivers/pci/switch/switch_discovery.c | 375 ++++++++++++++++++
drivers/pci/switch/switch_discovery.h | 44 ++
6 files changed, 494 insertions(+)
create mode 100644 Documentation/driver-api/pci/switch_discovery.rst
create mode 100644 drivers/pci/switch/switch_discovery.c
create mode 100644 drivers/pci/switch/switch_discovery.h

diff --git a/Documentation/driver-api/pci/switch_discovery.rst b/Documentation/driver-api/pci/switch_discovery.rst
new file mode 100644
index 000000000000..7c1476260e5e
--- /dev/null
+++ b/Documentation/driver-api/pci/switch_discovery.rst
@@ -0,0 +1,52 @@
+=================================
+Linux PCI Switch discovery module
+=================================
+
+Modern PCI switches support inter switch Peer-to-Peer(P2P) data transfer
+without using host resources. For example, Broadcom(PLX) PCIe Switches have a
+capability where a single physical switch can be divided up into multiple
+virtual switches at SOD. PCIe switch discovery module detects the virtual links
+between the switches that belong to the same physical switch.
+This allows user space applications to discover these virtual links that belong
+to the same physical switch and configure optimized data paths.
+
+Userspace Interface
+===================
+
+The module exposes sysfs entries for user space applications like MPI, NCCL,
+UCC, RCCL, HCCL, etc to discover the virtual switch links.
+
+Consider the below topology
+
+ Host root bridge
+ ---------------------------------------
+ | |
+ NIC1 --- PCI Switch1 --- Inter-switch link --- PCI Switch2 --- NIC2
+(af:00.0) (ad:00.0) (8b:00.0) (8d:00.0)
+ | |
+ GPU1 GPU2
+ (b0:00.0) (8e:00.0)
+ SERVER 1
+
+The simple topology above shows SERVER1, has Switch1 and Switch2 which are
+virtual switches that belong to the same physical switch that support
+Inter switch P2P.
+Switch1 and Switch2 have a GPU and NIC each connected.
+The module will detect the virtual P2P link existing between the two switches
+and create the sysfs entries as below.
+
+/sys/kernel/pci_switch_link/virtual_switch_links
+├── 0000:8b:00.0
+│   └── 0000:ad:00.0 -> ../0000:ad:00.0
+└── 0000:ad:00.0
+ └── 0000:8b:00.0 -> ../0000:8b:00.0
+
+The HPC/AI libraries that analyze the topology can decide the optimal data
+path like: NIC1->GPU1->GPU2->NIC1 which would have otherwise take a
+non-optimal path like NIC1->GPU1->GPU2->GPU1->NIC1.
+
+Enable P2P DMA to discover virtual links
+----------------------------------------
+The module also enhances :c:func:`pci_p2pdma_distance()` to determine a virtual
+link between the upstream PCI-to-PCI bridges of the devices and detect optimal
+path for applications using P2P DMA API.
diff --git a/MAINTAINERS b/MAINTAINERS
index 823387766a0c..b1bf3533ea6f 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -17359,6 +17359,19 @@ F: Documentation/driver-api/pci/p2pdma.rst
F: drivers/pci/p2pdma.c
F: include/linux/pci-p2pdma.h

+PCI SWITCH DISCOVERY
+M: Shivasharan S <shivasharan.srikanteshwara@xxxxxxxxxxxx>
+M: Sumanesh Samanta <sumanesh.samanta@xxxxxxxxxxxx>
+L: linux-pci@xxxxxxxxxxxxxxx
+S: Maintained
+Q: https://patchwork.kernel.org/project/linux-pci/list/
+B: https://bugzilla.kernel.org
+C: irc://irc.oftc.net/linux-pci
+T: git git://git.kernel.org/pub/scm/linux/kernel/git/pci/pci.git
+F: Documentation/driver-api/pci/switch_discovery.rst
+F: drivers/pci/switch/switch_discovery.c
+F: drivers/pci/switch/switch_discovery.h
+
PCI SUBSYSTEM
M: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
L: linux-pci@xxxxxxxxxxxxxxx
diff --git a/drivers/pci/switch/Kconfig b/drivers/pci/switch/Kconfig
index d370f4ce0492..fb4410153950 100644
--- a/drivers/pci/switch/Kconfig
+++ b/drivers/pci/switch/Kconfig
@@ -12,4 +12,13 @@ config PCI_SW_SWITCHTEC
devices. See <file:Documentation/driver-api/switchtec.rst> for more
information.

+config PCI_SW_DISCOVERY
+ depends on PCI
+ tristate "PCI Switch discovery module"
+ help
+ This kernel module discovers the PCI-to-PCI bridges of PCIe switches
+ and forms the virtual switch links if the bridges belong to the same
+ Physical switch. The switch links help to identify shorter distances
+ for P2P configurations.
+
endmenu
diff --git a/drivers/pci/switch/Makefile b/drivers/pci/switch/Makefile
index acd56d3b4a35..a3584b5146af 100644
--- a/drivers/pci/switch/Makefile
+++ b/drivers/pci/switch/Makefile
@@ -1,2 +1,3 @@
# SPDX-License-Identifier: GPL-2.0
obj-$(CONFIG_PCI_SW_SWITCHTEC) += switchtec.o
+obj-$(CONFIG_PCI_SW_DISCOVERY) += switch_discovery.o
diff --git a/drivers/pci/switch/switch_discovery.c b/drivers/pci/switch/switch_discovery.c
new file mode 100644
index 000000000000..a427d3885b1f
--- /dev/null
+++ b/drivers/pci/switch/switch_discovery.c
@@ -0,0 +1,375 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * PCI Switch Discovery module
+ *
+ * Copyright (c) 2024 Broadcom Inc.
+ *
+ * Authors: Broadcom Inc.
+ * Sumanesh Samanta <sumanesh.samanta@xxxxxxxxxxxx>
+ * Shivasharan S <shivasharan.srikanteshwara@xxxxxxxxxxxx>
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/sysfs.h>
+#include <linux/slab.h>
+#include <linux/rwsem.h>
+#include <linux/pci.h>
+#include <linux/vmalloc.h>
+#include "switch_discovery.h"
+
+static DECLARE_RWSEM(sw_disc_rwlock);
+static struct kobject *sw_disc_kobj, *sw_link_kobj;
+static struct kobject *sw_kobj[SWD_MAX_VIRT_SWITCH];
+static DECLARE_BITMAP(swdata_valid, SWD_MAX_VIRT_SWITCH);
+
+static struct switch_data *swdata;
+
+static int sw_disc_probe(void);
+static int sw_disc_create_sysfs_files(void);
+static bool brcm_sw_is_p2p_supported(struct pci_dev *pdev, char *serial_num);
+
+static inline bool sw_disc_is_supported_pdev(struct pci_dev *pdev)
+{
+ if ((pdev->vendor == PCI_VENDOR_ID_LSI) &&
+ ((pdev->device == PCI_DEVICE_ID_BRCM_PEX_89000_HLC) ||
+ (pdev->device == PCI_DEVICE_ID_BRCM_PEX_89000_LLC)))
+ return true;
+
+ return false;
+}
+
+static ssize_t sw_disc_show(struct kobject *kobj,
+ struct kobj_attribute *attr,
+ char *buf)
+{
+ int retval;
+
+ down_write(&sw_disc_rwlock);
+ retval = sw_disc_probe();
+ if (!retval) {
+ pr_debug("No new switch found\n");
+ goto exit_success;
+ }
+
+ retval = sw_disc_create_sysfs_files();
+ if (retval < 0) {
+ pr_err("Failed to create the sysfs entries, retval %d\n",
+ retval);
+ }
+
+exit_success:
+ up_write(&sw_disc_rwlock);
+ return sysfs_emit(buf, SWD_SCAN_DONE);
+}
+
+/* This function probes the PCIe devices for virtual links */
+static int sw_disc_probe(void)
+{
+ int i, bit;
+ struct pci_dev *pdev = NULL;
+ int topology_changed = 0;
+ DECLARE_BITMAP(sw_found_map, SWD_MAX_VIRT_SWITCH);
+
+ while ((pdev = pci_get_device(PCI_ANY_ID, PCI_ANY_ID, pdev)) != NULL) {
+ int sw_found;
+
+ /* Currently this function only traverses Broadcom
+ * PEX switches and determines the virtual SW links.
+ * Other Switch vendors can add their specific logic
+ * determine the virtual links.
+ */
+ if (!sw_disc_is_supported_pdev(pdev))
+ continue;
+
+ sw_found = -1;
+
+ for_each_set_bit(bit, swdata_valid, SWD_MAX_VIRT_SWITCH) {
+ if (swdata[bit].devfn == pdev->devfn &&
+ swdata[bit].bus == pdev->bus) {
+ sw_found = bit;
+ set_bit(sw_found, sw_found_map);
+ break;
+ }
+ }
+
+ if (sw_found != -1)
+ continue;
+
+ for (i = 0; i < SWD_MAX_VIRT_SWITCH; i++)
+ if (!swdata[i].bus)
+ break;
+
+ if (i >= SWD_MAX_VIRT_SWITCH) {
+ pr_err("Max switch exceeded\n");
+ break;
+ }
+
+ sw_found = i;
+
+ if (!brcm_sw_is_p2p_supported(pdev, (char *)&swdata[sw_found].serial_num))
+ continue;
+
+ /* Found a new switch which supports P2P */
+ swdata[sw_found].devfn = pdev->devfn;
+ swdata[sw_found].bus = pdev->bus;
+
+ topology_changed = 1;
+ set_bit(sw_found, sw_found_map);
+ set_bit(sw_found, swdata_valid);
+ }
+
+ /* handle device removal */
+ for_each_clear_bit(bit, sw_found_map, SWD_MAX_VIRT_SWITCH) {
+ if (test_bit(bit, swdata_valid)) {
+ memset(&swdata[bit], 0, sizeof(swdata[i]));
+ clear_bit(bit, swdata_valid);
+ topology_changed = 1;
+ }
+ }
+
+ return topology_changed;
+}
+
+/* Check the various config space registers of the Broadcom PCI device and
+ * return true if the device supports inter switch P2P.
+ * If P2P is supported, return the device serial number back to
+ * caller.
+ */
+bool brcm_sw_is_p2p_supported(struct pci_dev *pdev, char *serial_num)
+{
+ int base;
+ u32 cap_data1, cap_data2;
+ u16 vsec;
+ u32 vsec_data;
+
+ base = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_DSN);
+ if (!base) {
+ pr_debug("Failed to get extended capability bus %x devfn %x\n",
+ pdev->bus->number, pdev->devfn);
+ return false;
+ }
+
+ vsec = pci_find_vsec_capability(pdev, PCI_VENDOR_ID_LSI, 1);
+ if (!vsec) {
+ pr_debug("Failed to get VSEC bus %x devfn %x\n",
+ pdev->bus->number, pdev->devfn);
+ return false;
+ }
+
+ if (pci_pcie_type(pdev) != PCI_EXP_TYPE_UPSTREAM)
+ return false;
+
+ pci_read_config_dword(pdev, base + 8, &cap_data1);
+ pci_read_config_dword(pdev, base + 4, &cap_data2);
+
+ pci_read_config_dword(pdev, vsec + 12, &vsec_data);
+
+ pr_debug("Found Broadcom device bus 0x%x devfn 0x%x "
+ "Serial Number: 0x%x 0x%x, VSEC 0x%x\n",
+ pdev->bus->number, pdev->devfn,
+ cap_data1, cap_data2, vsec_data);
+
+ if (!SECURE_PART(cap_data1))
+ return false;
+
+ if (!(P2PMASK(vsec_data) & INTER_SWITCH_LINK))
+ return false;
+
+ if (serial_num)
+ snprintf(serial_num, SWD_MAX_CHAR, "%x%x", cap_data1, cap_data2);
+
+ return true;
+}
+
+static int sw_disc_create_sysfs_files(void)
+{
+ int i, j, retval;
+
+ for (i = 0; i < SWD_MAX_VIRT_SWITCH; i++) {
+ if (sw_kobj[i]) {
+ kobject_put(sw_kobj[i]);
+ sw_kobj[i] = NULL;
+ }
+ }
+
+ if (sw_link_kobj) {
+ kobject_put(sw_link_kobj);
+ sw_link_kobj = NULL;
+ }
+
+ sw_link_kobj = kobject_create_and_add(SWD_LINK_DIR_STRING, sw_disc_kobj);
+ if (!sw_link_kobj) {
+ pr_err("Failed to create pci link object\n");
+ return -ENOMEM;
+ }
+
+ for (i = 0; i < SWD_MAX_VIRT_SWITCH; i++) {
+ int segment, bus, device, function;
+ char bdf_i[SWD_MAX_CHAR];
+
+ if (!test_bit(i, swdata_valid))
+ continue;
+
+ segment = pci_domain_nr(swdata[i].bus);
+ bus = swdata[i].bus->number;
+ device = pci_ari_enabled(swdata[i].bus) ?
+ 0 : PCI_SLOT(swdata[i].devfn);
+ function = pci_ari_enabled(swdata[i].bus) ?
+ swdata[i].devfn : PCI_FUNC(swdata[i].devfn);
+ sprintf(bdf_i, "%04x:%02x:%02x.%x",
+ segment, bus, device, function);
+
+ for (j = i + 1; j < SWD_MAX_VIRT_SWITCH; j++) {
+ char bdf_j[SWD_MAX_CHAR];
+
+ if (!test_bit(j, swdata_valid))
+ continue;
+ segment = pci_domain_nr(swdata[j].bus);
+ bus = swdata[j].bus->number;
+ device = pci_ari_enabled(swdata[j].bus) ?
+ 0 : PCI_SLOT(swdata[j].devfn);
+ function = pci_ari_enabled(swdata[j].bus) ?
+ swdata[j].devfn : PCI_FUNC(swdata[j].devfn);
+ sprintf(bdf_j, "%04x:%02x:%02x.%x",
+ segment, bus, device, function);
+
+ if (strcmp(swdata[i].serial_num, swdata[j].serial_num) == 0) {
+ if (!sw_kobj[i]) {
+ sw_kobj[i] = kobject_create_and_add(bdf_i,
+ sw_link_kobj);
+ if (!sw_kobj[i]) {
+ pr_err("Failed to create sysfs entry for switch %s\n",
+ bdf_i);
+ }
+ }
+
+ if (!sw_kobj[j]) {
+ sw_kobj[j] = kobject_create_and_add(bdf_j,
+ sw_link_kobj);
+ if (!sw_kobj[j]) {
+ pr_err("Failed to create sysfs entry for switch %s\n",
+ bdf_j);
+ }
+ }
+
+ retval = sysfs_create_link(sw_kobj[i], sw_kobj[j], bdf_j);
+ if (retval)
+ pr_err("Error creating symlink %s and %s\n",
+ bdf_i, bdf_j);
+
+ retval = sysfs_create_link(sw_kobj[j], sw_kobj[i], bdf_i);
+ if (retval)
+ pr_err("Error creating symlink %s and %s\n",
+ bdf_j, bdf_i);
+ }
+ }
+ }
+
+ return 0;
+}
+
+/*
+ * Check if the two pci devices have virtual P2P link available.
+ * This function is used by the p2pdma to determine virtual
+ * links between the PCI-to-PCI bridges
+ */
+bool sw_disc_check_virtual_link(struct pci_dev *a,
+ struct pci_dev *b)
+{
+ char serial_num_a[SWD_MAX_CHAR], serial_num_b[SWD_MAX_CHAR];
+
+ /*
+ * Check if the PCIe devices support Virtual P2P links
+ */
+ if (!sw_disc_is_supported_pdev(a))
+ return false;
+
+ if (!sw_disc_is_supported_pdev(b))
+ return false;
+
+ if (brcm_sw_is_p2p_supported(a, serial_num_a) &&
+ brcm_sw_is_p2p_supported(b, serial_num_b))
+ if (!strcmp(serial_num_a, serial_num_b))
+ return true;
+
+ return false;
+}
+EXPORT_SYMBOL_GPL(sw_disc_check_virtual_link);
+
+static struct kobj_attribute sw_disc_attribute =
+ __ATTR(SWD_FILE_NAME_STRING, 0444, sw_disc_show, NULL);
+
+// Create attribute group
+static struct attribute *attrs[] = {
+ &sw_disc_attribute.attr,
+ NULL,
+};
+
+static struct attribute_group attr_group = {
+ .attrs = attrs,
+};
+
+static int __init sw_discovery_init(void)
+{
+ int i, retval;
+
+ for (i = 0; i < SWD_MAX_VIRT_SWITCH; i++)
+ sw_kobj[i] = NULL;
+
+ // Create "sw_disc" kobject
+ sw_disc_kobj = kobject_create_and_add(SWD_DIR_STRING, kernel_kobj);
+ if (!sw_disc_kobj) {
+ pr_err("Failed to create sw_disc_kobj\n");
+ return -ENOMEM;
+ }
+
+ retval = sysfs_create_group(sw_disc_kobj, &attr_group);
+ if (retval) {
+ pr_err("Cannot register sysfs attribute group\n");
+ kobject_put(sw_disc_kobj);
+ }
+
+ swdata = kzalloc(sizeof(swdata) * SWD_MAX_VIRT_SWITCH, GFP_KERNEL);
+ if (!swdata) {
+ sysfs_remove_group(sw_disc_kobj, &attr_group);
+ kobject_put(sw_disc_kobj);
+ return 0;
+ }
+
+ pr_info("Loading PCIe switch discovery module, version %s\n",
+ SWITCH_DISC_VERSION);
+
+ return 0;
+}
+
+static void __exit sw_discovery_exit(void)
+{
+ int i;
+
+ if (!swdata)
+ kfree(swdata);
+
+ for (i = 0; i < SWD_MAX_VIRT_SWITCH; i++) {
+ if (sw_kobj[i])
+ kobject_put(sw_kobj[i]);
+ }
+
+ // Remove kobject
+ if (sw_link_kobj)
+ kobject_put(sw_link_kobj);
+
+ sysfs_remove_group(sw_disc_kobj, &attr_group);
+ kobject_put(sw_disc_kobj);
+}
+
+module_init(sw_discovery_init);
+module_exit(sw_discovery_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Broadcom Inc.");
+MODULE_VERSION(SWITCH_DISC_VERSION);
+MODULE_DESCRIPTION("PCIe Switch Discovery Module");
diff --git a/drivers/pci/switch/switch_discovery.h b/drivers/pci/switch/switch_discovery.h
new file mode 100644
index 000000000000..b84f5d2e29ac
--- /dev/null
+++ b/drivers/pci/switch/switch_discovery.h
@@ -0,0 +1,44 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * PCI Switch Discovery module
+ *
+ * Copyright (c) 2024 Broadcom Inc.
+ *
+ * Authors: Broadcom Inc.
+ * Sumanesh Samanta <sumanesh.samanta@xxxxxxxxxxxx>
+ * Shivasharan S <shivasharan.srikanteshwara@xxxxxxxxxxxx>
+ */
+
+#ifndef PCI_SWITCH_DISC_H
+#define PCI_SWITCH_DISC_H
+
+#define SWD_MAX_SWITCH 32
+#define SWD_MAX_VER_PER_SWITCH 8
+
+#define SWD_MAX_VIRT_SWITCH (SWD_MAX_SWITCH * SWD_MAX_VER_PER_SWITCH)
+#define SWD_MAX_CHAR 16
+#define SWITCH_DISC_VERSION "0.1.1"
+#define SWD_DIR_STRING "pci_switch_link"
+#define SWD_LINK_DIR_STRING "virtual_switch_links"
+#define SWD_SCAN_DONE "done\n"
+
+#define SWD_FILE_NAME_STRING refresh_switch_toplogy
+
+/* Broadcom Vendor Specific definitions */
+#define PCI_VENDOR_ID_LSI 0x1000
+#define PCI_DEVICE_ID_BRCM_PEX_89000_HLC 0xC030
+#define PCI_DEVICE_ID_BRCM_PEX_89000_LLC 0xC034
+
+#define P2PMASK(x) (((x) & 0x300) >> 8)
+#define SECURE_PART(x) ((x) & 0x8)
+#define INTER_SWITCH_LINK 0x2
+
+struct switch_data {
+ int devfn;
+ struct pci_bus *bus;
+ char serial_num[SWD_MAX_CHAR];
+};
+
+bool sw_disc_check_virtual_link(struct pci_dev *a, struct pci_dev *b);
+
+#endif /* PCI_SWITCH_DISC_H */
--
2.43.0

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