PCI: Add pci_irqd_intx_xlate()

Legacy PCI INTx interrupts are represented in the PCI_INTERRUPT_PIN
register using the range 1-4, which matches our enum pci_interrupt_pin.
This is however not ideal for an IRQ domain, where with 4 interrupts we
would ideally have a domain of size 4 & hwirq numbers in the range 0-3.

Different PCI host controller drivers have handled this in different ways.
Of those under drivers/pci/ which register an INTx IRQ domain, we have:

  - pcie-altera uses the range 1-4 in device trees and an IRQ domain of
    size 5 to cover that range, with entry 0 wasted.

  - pcie-xilinx & pcie-xilinx-nwl use the range 1-4 in device trees but
    register an IRQ domain of size 4, which doesn't cover the hwirq=4/INTD
    case leading to that interrupt being broken.

  - pci-ftpci100 & pci-aardvark use the range 0-3 in both device trees & as
    hwirq numbering in the driver & IRQ domain.

In order to introduce some level of consistency in at least the hwirq
numbering used by the drivers & IRQ domains, this patch introduces a new
pci_irqd_intx_xlate() helper function which drivers using the 1-4 range in
device trees can assign as the xlate callback for their INTx IRQ domain.
This translates the 1-4 range into a 0-3 range, allowing us to use an IRQ
domain of size 4 & avoid a wasted entry. Further patches will make use of
this in drivers to allow them to use an IRQ domain of size 4 for legacy
INTx interrupts without breaking INTD.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

include/linux/pci.h

@@ -1416,6 +1416,38 @@ pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs,
 					      NULL);
 }
 
+/**
+ * pci_irqd_intx_xlate() - Translate PCI INTx value to an IRQ domain hwirq
+ * @d: the INTx IRQ domain
+ * @node: the DT node for the device whose interrupt we're translating
+ * @intspec: the interrupt specifier data from the DT
+ * @intsize: the number of entries in @intspec
+ * @out_hwirq: pointer at which to write the hwirq number
+ * @out_type: pointer at which to write the interrupt type
+ *
+ * Translate a PCI INTx interrupt number from device tree in the range 1-4, as
+ * stored in the standard PCI_INTERRUPT_PIN register, to a value in the range
+ * 0-3 suitable for use in a 4 entry IRQ domain. That is, subtract one from the
+ * INTx value to obtain the hwirq number.
+ *
+ * Returns 0 on success, or -EINVAL if the interrupt specifier is out of range.
+ */
+static inline int pci_irqd_intx_xlate(struct irq_domain *d,
+				      struct device_node *node,
+				      const u32 *intspec,
+				      unsigned int intsize,
+				      unsigned long *out_hwirq,
+				      unsigned int *out_type)
+{
+	const u32 intx = intspec[0];
+
+	if (intx < PCI_INTERRUPT_INTA || intx > PCI_INTERRUPT_INTD)
+		return -EINVAL;
+
+	*out_hwirq = intx - PCI_INTERRUPT_INTA;
+	return 0;
+}
+
 #ifdef CONFIG_PCIEPORTBUS
 extern bool pcie_ports_disabled;
 extern bool pcie_ports_auto;