[PATCH 2/2] PCI: Add pci=nodpc kernel boot option

[Yury Murashka <yurypm@arista.com>]

PCI DPC (Downstream Port Containment) support can be advertised by PCIe
devices, but it might not be fully supported in the firmware. On large
modular systems with a complex PCIe tree, enabling DPC could cause
unexpected behavior and side effects. Sometimes it would be nice to have
the option to keep the system in an unmodified state and be able to
handle PCIe errors from userspace.

Add pci=nodpc kernel boot option to disable PCI DPC. When this option
is set, DPC initialization, state save/restore, and recovery are all
skipped.

Signed-off-by: Yury Murashka <yurypm@arista.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  3 +++
 drivers/pci/pci.c                               |  2 ++
 drivers/pci/pci.h                               |  2 ++
 drivers/pci/pcie/dpc.c                          | 16 +++++++++++++---
 4 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index cfec12d37677..46a993c26dc0 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5065,6 +5065,9 @@ Kernel parameters
 				through ports 0xC000-0xCFFF).
 				See http://wiki.osdev.org/PCI for more info
 				on the configuration access mechanisms.
+		nodpc		[PCIE] If the PCIE_DPC kernel config parameter is
+				enabled, this kernel boot option can be used to
+				disable the use of PCIE DPC.
 		noaer		[PCIE] If the PCIEAER kernel config parameter is
 				enabled, this kernel boot option can be used to
 				disable the use of PCIE advanced error reporting.
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 1f71f9c773c4..2882c7bbb358 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -6723,6 +6723,8 @@ static int __init pci_setup(char *str)
 			} else if (!strncmp(str, "noats", 5)) {
 				pr_info("PCIe: ATS is disabled\n");
 				pcie_ats_disabled = true;
+			} else if (!strcmp(str, "nodpc")) {
+				pci_no_dpc();
 			} else if (!strcmp(str, "noaer")) {
 				pci_no_aer();
 			} else if (!strcmp(str, "noaer_recovery")) {
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index 7a79df0ae712..1b6f17dddf21 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -887,6 +887,7 @@ struct rcec_ea {
 #endif
 
 #ifdef CONFIG_PCIE_DPC
+void pci_no_dpc(void);
 void pci_save_dpc_state(struct pci_dev *dev);
 void pci_restore_dpc_state(struct pci_dev *dev);
 void pci_dpc_init(struct pci_dev *pdev);
@@ -895,6 +896,7 @@ pci_ers_result_t dpc_reset_link(struct pci_dev *pdev);
 bool pci_dpc_recovered(struct pci_dev *pdev);
 unsigned int dpc_tlp_log_len(struct pci_dev *dev);
 #else
+static inline void pci_no_dpc(void) { }
 static inline void pci_save_dpc_state(struct pci_dev *dev) { }
 static inline void pci_restore_dpc_state(struct pci_dev *dev) { }
 static inline void pci_dpc_init(struct pci_dev *pdev) { }
diff --git a/drivers/pci/pcie/dpc.c b/drivers/pci/pcie/dpc.c
index 2b779bd1d861..10d1a0e026d7 100644
--- a/drivers/pci/pcie/dpc.c
+++ b/drivers/pci/pcie/dpc.c
@@ -43,12 +43,19 @@ static const char * const rp_pio_error_string[] = {
 	"Memory Request Completion Timeout",		 /* Bit Position 18 */
 };
 
+static int pcie_dpc_disable;
+
+void pci_no_dpc(void)
+{
+	pcie_dpc_disable = 1;
+}
+
 void pci_save_dpc_state(struct pci_dev *dev)
 {
 	struct pci_cap_saved_state *save_state;
 	u16 *cap;
 
-	if (!pci_is_pcie(dev))
+	if (pcie_dpc_disable || !pci_is_pcie(dev))
 		return;
 
 	save_state = pci_find_saved_ext_cap(dev, PCI_EXT_CAP_ID_DPC);
@@ -64,7 +71,7 @@ void pci_restore_dpc_state(struct pci_dev *dev)
 	struct pci_cap_saved_state *save_state;
 	u16 *cap;
 
-	if (!pci_is_pcie(dev))
+	if (pcie_dpc_disable || !pci_is_pcie(dev))
 		return;
 
 	save_state = pci_find_saved_ext_cap(dev, PCI_EXT_CAP_ID_DPC);
@@ -104,7 +111,7 @@ bool pci_dpc_recovered(struct pci_dev *pdev)
 {
 	struct pci_host_bridge *host;
 
-	if (!pdev->dpc_cap)
+	if (pcie_dpc_disable || !pdev->dpc_cap)
 		return false;
 
 	/*
@@ -404,6 +411,9 @@ void pci_dpc_init(struct pci_dev *pdev)
 {
 	u16 cap;
 
+	if (pcie_dpc_disable)
+		return;
+
 	pdev->dpc_cap = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_DPC);
 	if (!pdev->dpc_cap)
 		return;
-- 
2.51.0