Some endpoint platforms cannot use platform MSI / GIC ITS to implement
EP-side doorbells. In those cases, EPF drivers cannot provide an
interrupt-driven doorbell and often fall back to polling.

Add an "embedded" doorbell backend that uses a controller-integrated
doorbell target (e.g. DesignWare integrated eDMA interrupt-emulation
doorbell).

The backend locates the doorbell register and a corresponding Linux IRQ
via the EPC aux-resource API. If the doorbell register is already
exposed via a fixed BAR mapping, provide BAR+offset. Otherwise provide
the DMA address returned by dma_map_resource() (which may be an IOVA
when an IOMMU is enabled) so EPF drivers can map it into BAR space.

When MSI doorbell allocation fails with -ENODEV,
pci_epf_alloc_doorbell() falls back to this embedded backend.

Suggested-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Koichiro Den <den@valinux.co.jp>
---
Changes in v12:
  - Use the new pci_epc_count_aux_resources()
  - Add TODO comment and defer multiple DOORBELL_MMIO resources per EPC
    case.

 drivers/pci/endpoint/pci-ep-msi.c | 135 +++++++++++++++++++++++++++++-
 include/linux/pci-epf.h           |   8 ++
 2 files changed, 140 insertions(+), 3 deletions(-)

diff --git a/drivers/pci/endpoint/pci-ep-msi.c b/drivers/pci/endpoint/pci-ep-msi.c
index 85fe46103220..0d720556205b 100644
--- a/drivers/pci/endpoint/pci-ep-msi.c
+++ b/drivers/pci/endpoint/pci-ep-msi.c
@@ -6,6 +6,8 @@
  * Author: Frank Li <Frank.Li@nxp.com>
  */
 
+#include <linux/align.h>
+#include <linux/cleanup.h>
 #include <linux/device.h>
 #include <linux/export.h>
 #include <linux/interrupt.h>
@@ -36,6 +38,113 @@ static void pci_epf_write_msi_msg(struct msi_desc *desc, struct msi_msg *msg)
 	pci_epc_put(epc);
 }
 
+static int pci_epf_alloc_doorbell_embedded(struct pci_epf *epf, u16 num_db)
+{
+	const struct pci_epc_aux_resource *doorbell = NULL;
+	struct pci_epf_doorbell_msg *msg;
+	struct pci_epc *epc = epf->epc;
+	size_t map_size = 0, off = 0;
+	dma_addr_t iova_base = 0;
+	phys_addr_t phys_base;
+	int count, ret, i;
+	u64 addr;
+
+	ret = pci_epc_count_aux_resources(epc, epf->func_no, epf->vfunc_no,
+					  &count);
+	if (ret == -EOPNOTSUPP)
+		return -ENODEV;
+	if (ret)
+		return ret;
+	if (!count)
+		return -ENODEV;
+
+	struct pci_epc_aux_resource *res __free(kfree) =
+				kcalloc(count, sizeof(*res), GFP_KERNEL);
+	if (!res)
+		return -ENOMEM;
+
+	ret = pci_epc_get_aux_resources(epc, epf->func_no, epf->vfunc_no,
+					res, count);
+	if (ret == -EOPNOTSUPP)
+		return -ENODEV;
+	if (ret)
+		return ret;
+
+	/* TODO: Support multiple DOORBELL_MMIO resources per EPC. */
+	for (i = 0; i < count; i++) {
+		if (res[i].type != PCI_EPC_AUX_DOORBELL_MMIO)
+			continue;
+
+		doorbell = &res[i];
+		break;
+	}
+	if (!doorbell)
+		return -ENODEV;
+	addr = doorbell->phys_addr;
+	if (!IS_ALIGNED(addr, sizeof(u32)))
+		return -EINVAL;
+
+	/*
+	 * Reuse the pre-exposed BAR window if available. Otherwise map the MMIO
+	 * doorbell resource here. Any required IOMMU mapping is handled
+	 * internally, matching the MSI doorbell semantics.
+	 */
+	if (doorbell->bar == NO_BAR) {
+		phys_base = addr & PAGE_MASK;
+		off = addr - phys_base;
+		map_size = PAGE_ALIGN(off + sizeof(u32));
+
+		iova_base = dma_map_resource(epc->dev.parent, phys_base,
+					     map_size, DMA_FROM_DEVICE, 0);
+		if (dma_mapping_error(epc->dev.parent, iova_base))
+			return -EIO;
+
+		addr = iova_base + off;
+	}
+
+	msg = kcalloc(num_db, sizeof(*msg), GFP_KERNEL);
+	if (!msg) {
+		ret = -ENOMEM;
+		goto err_unmap;
+	}
+
+	/*
+	 * Embedded doorbell backends (e.g. DesignWare eDMA interrupt emulation)
+	 * typically provide a single IRQ and do not offer per-doorbell
+	 * distinguishable address/data pairs. The EPC aux resource therefore
+	 * exposes one DOORBELL_MMIO entry (u.db_mmio.irq).
+	 *
+	 * Still, pci_epf_alloc_doorbell() allows requesting multiple doorbells.
+	 * For such backends we replicate the same address/data for each entry
+	 * and mark the IRQ as shared (IRQF_SHARED). Consumers must treat them
+	 * as equivalent "kick" doorbells.
+	 */
+	for (i = 0; i < num_db; i++)
+		msg[i] = (struct pci_epf_doorbell_msg) {
+			.msg.address_lo = (u32)addr,
+			.msg.address_hi = (u32)(addr >> 32),
+			.msg.data = doorbell->u.db_mmio.data,
+			.virq = doorbell->u.db_mmio.irq,
+			.irq_flags = IRQF_SHARED,
+			.type = PCI_EPF_DOORBELL_EMBEDDED,
+			.bar = doorbell->bar,
+			.offset = (doorbell->bar == NO_BAR) ? 0 :
+				  doorbell->bar_offset,
+			.iova_base = iova_base,
+			.iova_size = map_size,
+		};
+
+	epf->num_db = num_db;
+	epf->db_msg = msg;
+	return 0;
+
+err_unmap:
+	if (map_size)
+		dma_unmap_resource(epc->dev.parent, iova_base, map_size,
+				   DMA_FROM_DEVICE, 0);
+	return ret;
+}
+
 static int pci_epf_alloc_doorbell_msi(struct pci_epf *epf, u16 num_db)
 {
 	struct pci_epf_doorbell_msg *msg;
@@ -109,18 +218,38 @@ int pci_epf_alloc_doorbell(struct pci_epf *epf, u16 num_db)
 	if (!ret)
 		return 0;
 
-	dev_err(dev, "Failed to allocate doorbell: %d\n", ret);
-	return ret;
+	/*
+	 * Fall back to embedded doorbell only when platform MSI is unavailable
+	 * for this EPC.
+	 */
+	if (ret != -ENODEV)
+		return ret;
+
+	ret = pci_epf_alloc_doorbell_embedded(epf, num_db);
+	if (ret) {
+		dev_err(dev, "Failed to allocate doorbell: %d\n", ret);
+		return ret;
+	}
+
+	dev_info(dev, "Using embedded (DMA) doorbell fallback\n");
+	return 0;
 }
 EXPORT_SYMBOL_GPL(pci_epf_alloc_doorbell);
 
 void pci_epf_free_doorbell(struct pci_epf *epf)
 {
+	struct pci_epf_doorbell_msg *msg0;
+	struct pci_epc *epc = epf->epc;
+
 	if (!epf->db_msg)
 		return;
 
-	if (epf->db_msg[0].type == PCI_EPF_DOORBELL_MSI)
+	msg0 = &epf->db_msg[0];
+	if (msg0->type == PCI_EPF_DOORBELL_MSI)
 		platform_device_msi_free_irqs_all(epf->epc->dev.parent);
+	else if (msg0->type == PCI_EPF_DOORBELL_EMBEDDED && msg0->iova_size)
+		dma_unmap_resource(epc->dev.parent, msg0->iova_base,
+				   msg0->iova_size, DMA_FROM_DEVICE, 0);
 
 	kfree(epf->db_msg);
 	epf->db_msg = NULL;
diff --git a/include/linux/pci-epf.h b/include/linux/pci-epf.h
index cd747447a1ea..8a6c64a35890 100644
--- a/include/linux/pci-epf.h
+++ b/include/linux/pci-epf.h
@@ -171,6 +171,12 @@ enum pci_epf_doorbell_type {
  *       (NO_BAR if not)
  * @offset: offset within @bar for the doorbell target (valid iff
  *          @bar != NO_BAR)
+ * @iova_base: Internal: base DMA address returned by dma_map_resource() for the
+ *             embedded doorbell MMIO window (used only for unmapping). Valid
+ *             when @type is PCI_EPF_DOORBELL_EMBEDDED and @iova_size is
+ *             non-zero.
+ * @iova_size: Internal: size of the dma_map_resource() mapping at @iova_base.
+ *             Zero when no mapping was created (e.g. pre-exposed fixed BAR).
  */
 struct pci_epf_doorbell_msg {
 	struct msi_msg msg;
@@ -179,6 +185,8 @@ struct pci_epf_doorbell_msg {
 	enum pci_epf_doorbell_type type;
 	enum pci_barno bar;
 	resource_size_t offset;
+	dma_addr_t iova_base;
+	size_t iova_size;
 };
 
 /**
-- 
2.51.0