[PATCH v7 2/8] PCI: Store all PCIe Supported Link Speeds

Ilpo Järvinen
2024-09-10T12:27:47Z

PCIe bandwidth controller added by a subsequent commit will require
selecting PCIe Link Speeds that are lower than the Maximum Link Speed.

The struct pci_bus only stores max_bus_speed. Even if PCIe r6.1 sec
8.2.1 currently disallows gaps in supported Link Speeds, the
Implementation Note in PCIe r6.1 sec 7.5.3.18, recommends determining
supported Link Speeds using the Supported Link Speeds Vector in the
Link Capabilities 2 Register (when available) to "avoid software being
confused if a future specification defines Links that do not require
support for all slower speeds."

Reuse code in pcie_get_speed_cap() to add pcie_get_supported_speeds()
to query the Supported Link Speeds Vector of a PCIe device. The value
is taken directly from the Supported Link Speeds Vector or synthetized
from the Max Link Speed in the Link Capabilities Register when the Link
Capabilities 2 Register is not available.

The Supported Link Speeds Vector in the Link Capabilities Register 2
corresponds to the bus below on Root Ports and Downstream Ports,
whereas it corresponds to the bus above on Upstream Ports and
Endpoints (PCIe r6.1 sec 7.5.3.18):

"Supported Link Speeds Vector - This field indicates the
supported Link speed(s) of the associated Port."

Add supported_speeds into the struct pci_dev that caches the
Supported Link Speeds Vector.

supported_speeds contains a set of Link Speeds only in the case where
PCIe Link Speed can be determined. The Root Complex Integrated
Endpoints do not have a well-defined Link Speed because they do not
seem to implement either of the Link Capabilities Registers, which is
allowed by PCIe r6.1 sec 7.5.3 (the same limitation applies to
determining cur_bus_speed and max_bus_speed that are PCI_SPEED_UNKNOWN
in such case). This is of no concern from PCIe bandwidth controller
point of view because such devices are not attached into a PCIe Root
Port that could be controlled.

supported_speeds field keeps the extra reserved zero at the least
significant bit to match the Link Capabilities 2 Register layouting.

An attempt was made to store supported_speeds field into the struct
pci_bus as an intersection of the both ends of the Link, however, the
subordinate struct pci_bus is not available early enough. The Target
Speed quirk (in pcie_failed_link_retrain()) can run either during
initial scan or later requiring it to use the API PCIe bandwidth
controller provides to set the Target Link Speed in order to co-exist
with the bandwidth controller. When the Target Speed quirk is calling
the bandwidth controller during initial scan, the struct pci_bus is not
yet initialized. As such, storing supported_speeds into the struct
pci_bus is not viable.

Suggested-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
---
drivers/pci/pci.c | 58 ++++++++++++++++++++++++-----------
drivers/pci/probe.c | 3 ++
include/linux/pci.h | 11 ++++++-
include/uapi/linux/pci_regs.h | 1 +
4 files changed, 54 insertions(+), 19 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index d46652760b11..699531d9d112 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -6169,38 +6169,60 @@ u32 pcie_bandwidth_available(struct pci_dev *dev, struct pci_dev **limiting_dev,
EXPORT_SYMBOL(pcie_bandwidth_available);

/**
- * pcie_get_speed_cap - query for the PCI device's link speed capability
+ * pcie_get_supported_speeds - query Supported Link Speed Vector
* @dev: PCI device to query
*
- * Query the PCI device speed capability. Return the maximum link speed
- * supported by the device.
+ * Query @dev supported link speeds.
+ *
+ * Implementation Note in PCIe r6.0.1 sec 7.5.3.18 recommends determining
+ * supported link speeds using the Supported Link Speeds Vector in the Link
+ * Capabilities 2 Register (when available).
+ *
+ * Link Capabilities 2 was added in PCIe r3.0, sec 7.8.18.
+ *
+ * Without Link Capabilities 2, i.e., prior to PCIe r3.0, Supported Link
+ * Speeds field in Link Capabilities is used and only 2.5 GT/s and 5.0 GT/s
+ * speeds were defined.
+ *
+ * For @dev without Supported Link Speed Vector, the field is synthetized
+ * from the Max Link Speed field in the Link Capabilities Register.
+ *
+ * Return: Supported Link Speeds Vector
*/
-enum pci_bus_speed pcie_get_speed_cap(struct pci_dev *dev)
+u8 pcie_get_supported_speeds(struct pci_dev *dev)
{
u32 lnkcap2, lnkcap;
+ u8 speeds;

- /*
- * Link Capabilities 2 was added in PCIe r3.0, sec 7.8.18. The
- * implementation note there recommends using the Supported Link
- * Speeds Vector in Link Capabilities 2 when supported.
- *
- * Without Link Capabilities 2, i.e., prior to PCIe r3.0, software
- * should use the Supported Link Speeds field in Link Capabilities,
- * where only 2.5 GT/s and 5.0 GT/s speeds were defined.
- */
pcie_capability_read_dword(dev, PCI_EXP_LNKCAP2, &lnkcap2);
+ speeds = lnkcap2 & PCI_EXP_LNKCAP2_SLS;

/* PCIe r3.0-compliant */
- if (lnkcap2)
- return PCIE_LNKCAP2_SLS2SPEED(lnkcap2);
+ if (speeds)
+ return speeds;

pcie_capability_read_dword(dev, PCI_EXP_LNKCAP, &lnkcap);
+
+ /* Synthetize from the Max Link Speed field */
if ((lnkcap & PCI_EXP_LNKCAP_SLS) == PCI_EXP_LNKCAP_SLS_5_0GB)
- return PCIE_SPEED_5_0GT;
+ speeds = PCI_EXP_LNKCAP2_SLS_5_0GB | PCI_EXP_LNKCAP2_SLS_2_5GB;
else if ((lnkcap & PCI_EXP_LNKCAP_SLS) == PCI_EXP_LNKCAP_SLS_2_5GB)
- return PCIE_SPEED_2_5GT;
+ speeds = PCI_EXP_LNKCAP2_SLS_2_5GB;
+
+ return speeds;
+}

- return PCI_SPEED_UNKNOWN;
+/**
+ * pcie_get_speed_cap - query for the PCI device's link speed capability
+ * @dev: PCI device to query
+ *
+ * Query the PCI device speed capability.
+ *
+ * Return: the maximum link speed supported by the device.
+ */
+enum pci_bus_speed pcie_get_speed_cap(struct pci_dev *dev)
+{
+ return PCIE_LNKCAP2_SLS2SPEED(dev->supported_speeds);
}
EXPORT_SYMBOL(pcie_get_speed_cap);

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index b14b9876c030..761b0538afe6 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -1945,6 +1945,9 @@ int pci_setup_device(struct pci_dev *dev)

set_pcie_untrusted(dev);

+ if (pci_is_pcie(dev))
+ dev->supported_speeds = pcie_get_supported_speeds(dev);
+
/* "Unknown power state" */
dev->current_state = PCI_UNKNOWN;

diff --git a/include/linux/pci.h b/include/linux/pci.h
index 25362d577824..56ad75c4c994 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -303,6 +303,7 @@ enum pci_bus_speed {
PCI_SPEED_UNKNOWN = 0xff,
};

+u8 pcie_get_supported_speeds(struct pci_dev *dev);
enum pci_bus_speed pcie_get_speed_cap(struct pci_dev *dev);
enum pcie_link_width pcie_get_width_cap(struct pci_dev *dev);

@@ -318,7 +319,14 @@ struct pci_sriov;
struct pci_p2pdma;
struct rcec_ea;

-/* The pci_dev structure describes PCI devices */
+/* struct pci_dev - describes a PCI device
+ *
+ * @supported_speeds: PCIe Supported Link Speeds Vector (+ reserved 0 at
+ * LSB). 0 when the supported speeds cannot be
+ * determined (e.g., for Root Complex Integrated
+ * Endpoints without the relevant Capability
+ * Registers).
+ */
struct pci_dev {
struct list_head bus_list; /* Node in per-bus list */
struct pci_bus *bus; /* Bus this device is on */
@@ -518,6 +526,7 @@ struct pci_dev {
struct xarray doe_mbs; /* Data Object Exchange mailboxes */
#endif
u16 acs_cap; /* ACS Capability offset */
+ u8 supported_speeds; /* Supported Link Speeds Vector */
phys_addr_t rom; /* Physical address if not from BAR */
size_t romlen; /* Length if not from BAR */
/*
diff --git a/include/uapi/linux/pci_regs.h b/include/uapi/linux/pci_regs.h
index 94c00996e633..b8140fc184cc 100644
--- a/include/uapi/linux/pci_regs.h
+++ b/include/uapi/linux/pci_regs.h
@@ -676,6 +676,7 @@
#define PCI_EXP_DEVSTA2 0x2a /* Device Status 2 */
#define PCI_CAP_EXP_RC_ENDPOINT_SIZEOF_V2 0x2c /* end of v2 EPs w/o link */
#define PCI_EXP_LNKCAP2 0x2c /* Link Capabilities 2 */
+#define PCI_EXP_LNKCAP2_SLS 0x000000fe /* Supported Link Speeds Vector */
#define PCI_EXP_LNKCAP2_SLS_2_5GB 0x00000002 /* Supported Speed 2.5GT/s */
#define PCI_EXP_LNKCAP2_SLS_5_0GB 0x00000004 /* Supported Speed 5GT/s */
#define PCI_EXP_LNKCAP2_SLS_8_0GB 0x00000008 /* Supported Speed 8GT/s */
--
2.39.2