[PATCH v8 9/9] PCI: endpoint: pci-ep-msi: Add embedded eDMA doorbell fallback

[Koichiro Den]

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 physical address 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.

Signed-off-by: Koichiro Den <den at valinux.co.jp>
---
Changes since v7:
  - Use designated initializer for consistent styling.
  - Clean up the pci_epf_alloc_doorbell() error path.

 drivers/pci/endpoint/pci-ep-msi.c | 96 ++++++++++++++++++++++++++++++-
 1 file changed, 94 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/endpoint/pci-ep-msi.c b/drivers/pci/endpoint/pci-ep-msi.c
index 50badffa9d72..66f5a68c81b0 100644
--- a/drivers/pci/endpoint/pci-ep-msi.c
+++ b/drivers/pci/endpoint/pci-ep-msi.c
@@ -6,6 +6,7 @@
  * Author: Frank Li <Frank.Li at nxp.com>
  */
 
+#include <linux/cleanup.h>
 #include <linux/device.h>
 #include <linux/export.h>
 #include <linux/interrupt.h>
@@ -36,6 +37,84 @@ 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;
+	struct device *dev = &epf->dev;
+	int count, ret, i;
+	u64 addr;
+
+	count = pci_epc_get_aux_resources(epc, epf->func_no, epf->vfunc_no,
+					  NULL, 0);
+	if (count == -EOPNOTSUPP || count == 0)
+		return -ENODEV;
+	if (count < 0)
+		return count;
+
+	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 || ret == 0)
+		return -ENODEV;
+	if (ret < 0)
+		return ret;
+
+	count = ret;
+
+	for (i = 0; i < count; i++) {
+		if (res[i].type == PCI_EPC_AUX_DOORBELL_MMIO) {
+			if (doorbell) {
+				dev_warn(dev,
+					 "Duplicate DOORBELL_MMIO resource found\n");
+				continue;
+			}
+			doorbell = &res[i];
+		}
+	}
+	if (!doorbell)
+		return -ENODEV;
+
+	msg = kcalloc(num_db, sizeof(*msg), GFP_KERNEL);
+	if (!msg)
+		return -ENOMEM;
+
+	addr = doorbell->phys_addr;
+
+	/*
+	 * 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,
+		};
+
+	epf->num_db = num_db;
+	epf->db_msg = msg;
+	return 0;
+}
+
 static int pci_epf_alloc_doorbell_msi(struct pci_epf *epf, u16 num_db)
 {
 	struct pci_epf_doorbell_msg *msg;
@@ -109,8 +188,21 @@ 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);
 
-- 
2.51.0