PCI: endpoint: Add RC-to-EP doorbell support using platform MSI controller

Implement the doorbell feature by mapping the EP's MSI interrupt controller
message address to a dedicated BAR.

The EPF driver should pass the actual message data to be written to the
message address by the host through implementation-specific logic.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
[mani: minor code cleanups and reworded commit message]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
[bhelgaas: fix kernel-doc]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Niklas Cassel <cassel@kernel.org>
Link: https://patch.msgid.link/20250710-ep-msi-v21-3-57683fc7fb25@nxp.com

drivers/pci/endpoint/Kconfig

@@ -28,6 +28,14 @@ config PCI_ENDPOINT_CONFIGFS
 	   configure the endpoint function and used to bind the
 	   function with an endpoint controller.
 
+config PCI_ENDPOINT_MSI_DOORBELL
+	bool "PCI Endpoint MSI Doorbell Support"
+	depends on PCI_ENDPOINT && GENERIC_MSI_IRQ
+	help
+	  This enables the EP's MSI interrupt controller to function as a
+	  doorbell. The RC can trigger doorbell in EP by writing data to a
+	  dedicated BAR, which the EP maps to the controller's message address.
+
 source "drivers/pci/endpoint/functions/Kconfig"
 
 endmenu

drivers/pci/endpoint/Makefile

@@ -6,3 +6,4 @@
 obj-$(CONFIG_PCI_ENDPOINT_CONFIGFS)	+= pci-ep-cfs.o
 obj-$(CONFIG_PCI_ENDPOINT)		+= pci-epc-core.o pci-epf-core.o\
 					   pci-epc-mem.o functions/
+obj-$(CONFIG_PCI_ENDPOINT_MSI_DOORBELL)	+= pci-ep-msi.o

drivers/pci/endpoint/pci-ep-msi.c

@@ -0,0 +1,92 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * PCI Endpoint *Controller* (EPC) MSI library
+ *
+ * Copyright (C) 2025 NXP
+ * Author: Frank Li <Frank.Li@nxp.com>
+ */
+
+#include <linux/device.h>
+#include <linux/export.h>
+#include <linux/irqdomain.h>
+#include <linux/module.h>
+#include <linux/msi.h>
+#include <linux/of_irq.h>
+#include <linux/pci-epc.h>
+#include <linux/pci-epf.h>
+#include <linux/pci-ep-cfs.h>
+#include <linux/pci-ep-msi.h>
+#include <linux/slab.h>
+
+static void pci_epf_write_msi_msg(struct msi_desc *desc, struct msi_msg *msg)
+{
+	struct pci_epc *epc;
+	struct pci_epf *epf;
+
+	epc = pci_epc_get(dev_name(msi_desc_to_dev(desc)));
+	if (!epc)
+		return;
+
+	epf = list_first_entry_or_null(&epc->pci_epf, struct pci_epf, list);
+
+	if (epf && epf->db_msg && desc->msi_index < epf->num_db)
+		memcpy(&epf->db_msg[desc->msi_index].msg, msg, sizeof(*msg));
+
+	pci_epc_put(epc);
+}
+
+int pci_epf_alloc_doorbell(struct pci_epf *epf, u16 num_db)
+{
+	struct pci_epc *epc = epf->epc;
+	struct device *dev = &epf->dev;
+	struct irq_domain *domain;
+	void *msg;
+	int ret;
+	int i;
+
+	/* TODO: Multi-EPF support */
+	if (list_first_entry_or_null(&epc->pci_epf, struct pci_epf, list) != epf) {
+		dev_err(dev, "MSI doorbell doesn't support multiple EPF\n");
+		return -EINVAL;
+	}
+
+	domain = of_msi_map_get_device_domain(epc->dev.parent, 0,
+					      DOMAIN_BUS_PLATFORM_MSI);
+	if (!domain) {
+		dev_err(dev, "Can't find MSI domain for EPC\n");
+		return -ENODEV;
+	}
+
+	dev_set_msi_domain(epc->dev.parent, domain);
+
+	msg = kcalloc(num_db, sizeof(struct pci_epf_doorbell_msg), GFP_KERNEL);
+	if (!msg)
+		return -ENOMEM;
+
+	epf->num_db = num_db;
+	epf->db_msg = msg;
+
+	ret = platform_device_msi_init_and_alloc_irqs(epc->dev.parent, num_db,
+						      pci_epf_write_msi_msg);
+	if (ret) {
+		dev_err(dev, "Failed to allocate MSI\n");
+		kfree(msg);
+		return ret;
+	}
+
+	for (i = 0; i < num_db; i++)
+		epf->db_msg[i].virq = msi_get_virq(epc->dev.parent, i);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(pci_epf_alloc_doorbell);
+
+void pci_epf_free_doorbell(struct pci_epf *epf)
+{
+	platform_device_msi_free_irqs_all(epf->epc->dev.parent);
+
+	kfree(epf->db_msg);
+	epf->db_msg = NULL;
+	epf->num_db = 0;
+}
+EXPORT_SYMBOL_GPL(pci_epf_free_doorbell);

include/linux/pci-ep-msi.h

@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * PCI Endpoint *Function* side MSI header file
+ *
+ * Copyright (C) 2024 NXP
+ * Author: Frank Li <Frank.Li@nxp.com>
+ */
+
+#ifndef __PCI_EP_MSI__
+#define __PCI_EP_MSI__
+
+struct pci_epf;
+
+#ifdef CONFIG_PCI_ENDPOINT_MSI_DOORBELL
+int pci_epf_alloc_doorbell(struct pci_epf *epf, u16 nums);
+void pci_epf_free_doorbell(struct pci_epf *epf);
+#else
+static inline int pci_epf_alloc_doorbell(struct pci_epf *epf, u16 nums)
+{
+	return -ENODATA;
+}
+
+static inline void pci_epf_free_doorbell(struct pci_epf *epf)
+{
+}
+#endif /* CONFIG_GENERIC_MSI_IRQ */
+
+#endif /* __PCI_EP_MSI__ */

include/linux/pci-epf.h

@@ -12,6 +12,7 @@
 #include <linux/configfs.h>
 #include <linux/device.h>
 #include <linux/mod_devicetable.h>
+#include <linux/msi.h>
 #include <linux/pci.h>
 
 struct pci_epf;
@@ -128,6 +129,16 @@ struct pci_epf_bar {
 	int		flags;
 };
 
+/**
+ * struct pci_epf_doorbell_msg - represents doorbell message
+ * @msg: MSI message
+ * @virq: IRQ number of this doorbell MSI message
+ */
+struct pci_epf_doorbell_msg {
+	struct msi_msg msg;
+	int virq;
+};
+
 /**
  * struct pci_epf - represents the PCI EPF device
  * @dev: the PCI EPF device
@@ -155,6 +166,8 @@ struct pci_epf_bar {
  * @vfunction_num_map: bitmap to manage virtual function number
  * @pci_vepf: list of virtual endpoint functions associated with this function
  * @event_ops: callbacks for capturing the EPC events
+ * @db_msg: data for MSI from RC side
+ * @num_db: number of doorbells
  */
 struct pci_epf {
 	struct device		dev;
@@ -185,6 +198,8 @@ struct pci_epf {
 	unsigned long		vfunction_num_map;
 	struct list_head	pci_vepf;
 	const struct pci_epc_event_ops *event_ops;
+	struct pci_epf_doorbell_msg *db_msg;
+	u16 num_db;
 };
 
 /**