x86/PCI: Fix ALi M1487 (IBC) PIRQ router link value interpretation

Fix an issue with commit 1ce849c7 ("x86/PCI: Add support for the ALi 
M1487 (IBC) PIRQ router") and correct ALi M1487 (IBC) PIRQ router link 
value (`pirq' cookie) interpretation according to findings in the BIOS.

Credit to Nikolai Zhubr for the detective work as to the bit layout.

Fixes: 1ce849c7 ("x86/PCI: Add support for the ALi M1487 (IBC) PIRQ router")
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2203310013270.44113@angie.orcam.me.uk

arch/x86/pci/irq.c

@@ -337,6 +337,15 @@ static void write_pc_conf_nybble(u8 base, u8 index, u8 val)
 	pc_conf_set(reg, x);
 }
 
+/*
+ * FinALi pirq rules are as follows:
+ *
+ * - bit 0 selects between INTx Routing Table Mapping Registers,
+ *
+ * - bit 3 selects the nibble within the INTx Routing Table Mapping Register,
+ *
+ * - bits 7:4 map to bits 3:0 of the PCI INTx Sensitivity Register.
+ */
 static int pirq_finali_get(struct pci_dev *router, struct pci_dev *dev,
 			   int pirq)
 {
@@ -344,11 +353,13 @@ static int pirq_finali_get(struct pci_dev *router, struct pci_dev *dev,
 		0, 9, 3, 10, 4, 5, 7, 6, 0, 11, 0, 12, 0, 14, 0, 15
 	};
 	unsigned long flags;
+	u8 index;
 	u8 x;
 
+	index = (pirq & 1) << 1 | (pirq & 8) >> 3;
 	raw_spin_lock_irqsave(&pc_conf_lock, flags);
 	pc_conf_set(PC_CONF_FINALI_LOCK, PC_CONF_FINALI_LOCK_KEY);
-	x = irqmap[read_pc_conf_nybble(PC_CONF_FINALI_PCI_INTX_RT1, pirq - 1)];
+	x = irqmap[read_pc_conf_nybble(PC_CONF_FINALI_PCI_INTX_RT1, index)];
 	pc_conf_set(PC_CONF_FINALI_LOCK, 0);
 	raw_spin_unlock_irqrestore(&pc_conf_lock, flags);
 	return x;
@@ -362,13 +373,15 @@ static int pirq_finali_set(struct pci_dev *router, struct pci_dev *dev,
 	};
 	u8 val = irqmap[irq];
 	unsigned long flags;
+	u8 index;
 
 	if (!val)
 		return 0;
 
+	index = (pirq & 1) << 1 | (pirq & 8) >> 3;
 	raw_spin_lock_irqsave(&pc_conf_lock, flags);
 	pc_conf_set(PC_CONF_FINALI_LOCK, PC_CONF_FINALI_LOCK_KEY);
-	write_pc_conf_nybble(PC_CONF_FINALI_PCI_INTX_RT1, pirq - 1, val);
+	write_pc_conf_nybble(PC_CONF_FINALI_PCI_INTX_RT1, index, val);
 	pc_conf_set(PC_CONF_FINALI_LOCK, 0);
 	raw_spin_unlock_irqrestore(&pc_conf_lock, flags);
 	return 1;
@@ -377,7 +390,7 @@ static int pirq_finali_set(struct pci_dev *router, struct pci_dev *dev,
 static int pirq_finali_lvl(struct pci_dev *router, struct pci_dev *dev,
 			   int pirq, int irq)
 {
-	u8 mask = ~(1u << (pirq - 1));
+	u8 mask = ~((pirq & 0xf0u) >> 4);
 	unsigned long flags;
 	u8 trig;