[PATCH] PCI: dwc: fall back to legacy IRQs when multiple devices are attached

[Lucas Stach <l.stach@pengutronix.de>]

The DWC host does not support legacy PCI IRQs and MSIs at the same time,
so we need to fall back to using only legacy IRQs if there is a chance
that multiple devices with differing MSI capabilities are connected to
the host. The only configuration where MSIs can be safely used is when
the device below the host bridge is not a bridge, i.e. the only device
connected to this host.

By disallowing MSI allocation when multiple devices might be attached
we get those configurations in a working state. The only configurations
that depend on MSIs being available, that I am aware of, are some
embedded devices with a PCIe attached FPGA, that is incapable of
generating PCI legacy IRQs. Those are kept working by allowing MSIs
when only a single device is attached to the host.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
---
 drivers/pci/dwc/pcie-designware-host.c | 24 ++++++++++++++++++++++--
 drivers/pci/dwc/pcie-designware.h      |  1 +
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/dwc/pcie-designware-host.c b/drivers/pci/dwc/pcie-designware-host.c
index 5ba334938b52..be2b2762c944 100644
--- a/drivers/pci/dwc/pcie-designware-host.c
+++ b/drivers/pci/dwc/pcie-designware-host.c
@@ -92,6 +92,8 @@ void dw_pcie_msi_init(struct pcie_port *pp)
 			    (u32)(msi_target & 0xffffffff));
 	dw_pcie_wr_own_conf(pp, PCIE_MSI_ADDR_HI, 4,
 			    (u32)(msi_target >> 32 & 0xffffffff));
+
+	pp->msi_disabled = false;
 }
 
 static void dw_pcie_msi_clear_irq(struct pcie_port *pp, int irq)
@@ -136,9 +138,11 @@ static void dw_pcie_msi_set_irq(struct pcie_port *pp, int irq)
 static int assign_irq(int no_irqs, struct msi_desc *desc, int *pos)
 {
 	int irq, pos0, i;
-	struct pcie_port *pp;
+	struct pcie_port *pp = (struct pcie_port *)msi_desc_to_pci_sysdata(desc);
+
+	if (pp->msi_disabled)
+		return -EINVAL;
 
-	pp = (struct pcie_port *)msi_desc_to_pci_sysdata(desc);
 	pos0 = bitmap_find_free_region(pp->msi_irq_in_use, MAX_MSI_IRQS,
 				       order_base_2(no_irqs));
 	if (pos0 < 0)
@@ -410,6 +414,22 @@ int dw_pcie_host_init(struct pcie_port *pp)
 		goto error;
 	}
 
+	/*
+	 * The DWC host does not support legacy PCI IRQs and MSIs at the same
+	 * time, so we need to fall back to using only legacy IRQs if there is
+	 * a chance that multiple devices with differing MSI capabilities are
+	 * connected to the host. The only configuration where MSIs can be
+	 * safely used is when the device below the host bridge is not a bridge,
+	 * i.e. the only device connected to this host.
+	 */
+	child = list_first_entry(&bus->children, struct pci_bus, node);
+	if (!list_empty(&child->devices)) {
+		struct pci_dev *dev = list_first_entry(&child->devices,
+						       struct pci_dev, bus_list);
+		if (pci_is_bridge(dev))
+			pp->msi_disabled = true;
+	}
+
 	if (pp->ops->scan_bus)
 		pp->ops->scan_bus(pp);
 
diff --git a/drivers/pci/dwc/pcie-designware.h b/drivers/pci/dwc/pcie-designware.h
index cd3b8713fe50..741bc31f8947 100644
--- a/drivers/pci/dwc/pcie-designware.h
+++ b/drivers/pci/dwc/pcie-designware.h
@@ -140,6 +140,7 @@ struct pcie_port {
 	struct irq_domain	*irq_domain;
 	unsigned long		msi_data;
 	DECLARE_BITMAP(msi_irq_in_use, MAX_MSI_IRQS);
+	bool			msi_disabled;
 };
 
 struct dw_pcie_ops {
-- 
2.11.0