[PATCH V4 10/12] PCI/TPH: Add pci=nostmode to force TPH No ST Mode

[Wei Huang <wei.huang2@xxxxxxx>]

When "No ST mode" is enabled, endpoint devices can generate TPH headers
but with all steering tags treated as zero. A steering tag of zero is
interpreted as "using the default policy" by the root complex. This is
essential to quantify the benefit of Steering Tags for some given
workloads.

Co-developed-by: Eric Van Tassell <Eric.VanTassell@xxxxxxx>
Signed-off-by: Eric Van Tassell <Eric.VanTassell@xxxxxxx>
Signed-off-by: Wei Huang <wei.huang2@xxxxxxx>
Reviewed-by: Ajit Khaparde <ajit.khaparde@xxxxxxxxxxxx>
Reviewed-by: Somnath Kotur <somnath.kotur@xxxxxxxxxxxx>
Reviewed-by: Andy Gospodarek <andrew.gospodarek@xxxxxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt |  3 +++
 drivers/pci/pci.c                               |  2 ++
 drivers/pci/pci.h                               |  2 ++
 drivers/pci/pcie/tph.c                          | 12 ++++++++++++
 4 files changed, 19 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f1384c7b59c9..ed2ee97cf7fb 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4617,6 +4617,9 @@
 		nomio		[S390] Do not use MIO instructions.
 		norid		[S390] ignore the RID field and force use of
 				one PCI domain per PCI function
+		nostmode	[PCIE] If PCIe TPH Processing Hints (TPH) is
+				enabled, this kernel option forces all Steering
+				Tags to be treated as zero (aka "No ST Mode").
 
 	pcie_aspm=	[PCIE] Forcibly enable or ignore PCIe Active State Power
 			Management.
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 1e4960994b1a..88aabac354c0 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -6870,6 +6870,8 @@ static int __init pci_setup(char *str)
 				pci_no_domains();
 			} else if (!strncmp(str, "noari", 5)) {
 				pcie_ari_disabled = true;
+			} else if (!strncmp(str, "nostmode", 8)) {
+				pci_tph_set_nostmode();
 			} else if (!strncmp(str, "cbiosize=", 9)) {
 				pci_cardbus_io_size = memparse(str + 9, &str);
 			} else if (!strncmp(str, "cbmemsize=", 10)) {
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index d7c7f86e8705..54d74f5ff861 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -574,10 +574,12 @@ static inline int pci_iov_bus_range(struct pci_bus *bus)
 #ifdef CONFIG_PCIE_TPH
 void pci_restore_tph_state(struct pci_dev *dev);
 void pci_save_tph_state(struct pci_dev *dev);
+void pci_tph_set_nostmode(void);
 void pci_tph_init(struct pci_dev *dev);
 #else
 static inline void pci_restore_tph_state(struct pci_dev *dev) { }
 static inline void pci_save_tph_state(struct pci_dev *dev) { }
+static inline void pci_tph_set_nostmode(void) { }
 static inline void pci_tph_init(struct pci_dev *dev) { }
 #endif
 
diff --git a/drivers/pci/pcie/tph.c b/drivers/pci/pcie/tph.c
index b228ef5b7948..f723352adcf5 100644
--- a/drivers/pci/pcie/tph.c
+++ b/drivers/pci/pcie/tph.c
@@ -48,6 +48,8 @@ union st_info {
 	u64 value;
 };
 
+static bool pci_tph_nostmode;
+
 static u16 tph_extract_tag(enum tph_mem_type mem_type, u8 req_type,
 			   union st_info *info)
 {
@@ -433,6 +435,10 @@ int pcie_enable_tph(struct pci_dev *pdev, int mode)
 		return -EINVAL;
 	}
 
+	/* Honor "nostmode" kernel parameter */
+	if (pci_tph_nostmode)
+		pdev->tph_mode = PCI_TPH_NO_ST_MODE;
+
 	/* Get req_type supported by device and its Root Port */
 	reg = pci_read_config_dword(pdev, pdev->tph_cap + PCI_TPH_CAP, &reg);
 	if (FIELD_GET(PCI_TPH_CAP_EXT_TPH, reg))
@@ -545,6 +551,12 @@ void pci_save_tph_state(struct pci_dev *pdev)
 	}
 }
 
+void pci_tph_set_nostmode(void)
+{
+	pci_tph_nostmode = true;
+	pr_info("PCIe TPH No ST Mode is enabled\n");
+}
+
 void pci_tph_init(struct pci_dev *pdev)
 {
 	pdev->tph_cap = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_TPH);
-- 
2.45.1