[PATCH v2] PCI/MSI: Improve MSI alias detection

[Robin Murphy <robin.murphy@xxxxxxx>]

Currently, we handle all DMA aliases equally when calculating MSI
requester IDs for the generic infrastructure. This turns out to be the
wrong thing to do in the face of pure DMA quirks like those of Marvell
SATA cards, where in the usual case the last thing seen in the alias
walk is the DMA phantom function: we end up configuring the MSI
doorbell to expect that alias, then find we have no interrupts since
the MSI writes still come from the 'real' RID, thus get filtered out
and ignored.

Improve the alias walk to only account for the topological aliases that
matter, based on the logic from the Intel IRQ remapping code.

Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
---

v2: Properly document the rationale.

 drivers/pci/msi.c | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
index 253d92409bb3..2f0dd02d78b7 100644
--- a/drivers/pci/msi.c
+++ b/drivers/pci/msi.c
@@ -1458,13 +1458,30 @@ struct irq_domain *pci_msi_create_irq_domain(struct fwnode_handle *fwnode,
 }
 EXPORT_SYMBOL_GPL(pci_msi_create_irq_domain);
 
+/*
+ * Users of the generic MSI infrastructure expect a device to have a single ID,
+ * so with DMA aliases we have to pick the least-worst compromise. Devices with
+ * DMA phantom functions tend to still emit MSIs from the real function number,
+ * so we ignore those and only consider topological aliases where either the
+ * alias device or RID appears on a different bus number. We also make the
+ * reasonable assumption that bridges are walked in an upstream direction (so
+ * the last one seen wins), and the much braver assumption that the most likely
+ * case is that of PCI->PCIe so we should always use the alias RID. This echoes
+ * the logic from intel_irq_remapping's set_msi_sid(), which presumably works
+ * well enough in practice; in the face of the horrible PCIe<->PCI-X conditions
+ * for taking ownership all we can really do is close our eyes and hope...
+ */
 static int get_msi_id_cb(struct pci_dev *pdev, u16 alias, void *data)
 {
 	u32 *pa = data;
+	u8 bus = PCI_BUS_NUM(*pa);
+
+	if (pdev->bus->number != bus || PCI_BUS_NUM(alias) != bus)
+		*pa = alias;
 
-	*pa = alias;
 	return 0;
 }
+
 /**
  * pci_msi_domain_get_msi_rid - Get the MSI requester id (RID)
  * @domain:	The interrupt domain
@@ -1478,7 +1495,7 @@ static int get_msi_id_cb(struct pci_dev *pdev, u16 alias, void *data)
 u32 pci_msi_domain_get_msi_rid(struct irq_domain *domain, struct pci_dev *pdev)
 {
 	struct device_node *of_node;
-	u32 rid = 0;
+	u32 rid = PCI_DEVID(pdev->bus->number, pdev->devfn);
 
 	pci_for_each_dma_alias(pdev, get_msi_id_cb, &rid);
 
@@ -1494,14 +1511,14 @@ u32 pci_msi_domain_get_msi_rid(struct irq_domain *domain, struct pci_dev *pdev)
  * @pdev:	The PCI device
  *
  * Use the firmware data to find a device-specific MSI domain
- * (i.e. not one that is ste as a default).
+ * (i.e. not one that is set as a default).
  *
- * Returns: The coresponding MSI domain or NULL if none has been found.
+ * Returns: The corresponding MSI domain or NULL if none has been found.
  */
 struct irq_domain *pci_msi_get_device_domain(struct pci_dev *pdev)
 {
 	struct irq_domain *dom;
-	u32 rid = 0;
+	u32 rid = PCI_DEVID(pdev->bus->number, pdev->devfn);
 
 	pci_for_each_dma_alias(pdev, get_msi_id_cb, &rid);
 	dom = of_msi_map_get_device_domain(&pdev->dev, rid);
-- 
2.12.2.dirty