Documentation: Update IRQ-domain.txt to document irq_domain_mapping

irq_domain_mapping is a rather useful tool to understand how IRqs
are mapped in irqdomains, and yet it is not documented anywhere.
Let's address this.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Link: http://lkml.kernel.org/r/20170512115538.10767-5-marc.zyngier@arm.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Documentation/IRQ-domain.txt

@@ -231,5 +231,42 @@ needs to:
 4) No need to implement irq_domain_ops.map and irq_domain_ops.unmap,
    they are unused with hierarchy irq_domain.
 
-Hierarchy irq_domain may also be used to support other architectures,
-such as ARM, ARM64 etc.
+Hierarchy irq_domain is in no way x86 specific, and is heavily used to
+support other architectures, such as ARM, ARM64 etc.
+
+=== Debugging ===
+
+If you switch on CONFIG_IRQ_DOMAIN_DEBUG (which depends on
+CONFIG_IRQ_DOMAIN and CONFIG_DEBUG_FS), you will find a new file in
+your debugfs mount point, called irq_domain_mapping. This file
+contains a live snapshot of all the IRQ domains in the system:
+
+ name              mapped  linear-max  direct-max  devtree-node
+ pl061                  8           8           0  /smb/gpio@e0080000
+ pl061                  8           8           0  /smb/gpio@e1050000
+ pMSI                   0           0           0  /interrupt-controller@e1101000/v2m@e0080000
+ MSI                   37           0           0  /interrupt-controller@e1101000/v2m@e0080000
+ GICv2m                37           0           0  /interrupt-controller@e1101000/v2m@e0080000
+ GICv2                448         448           0  /interrupt-controller@e1101000
+
+it also iterates over the interrupts to display their mapping in the
+domains, and makes the domain stacking visible:
+
+
+irq    hwirq    chip name        chip data           active  type            domain
+    1  0x00019  GICv2            0xffff00000916bfd8     *    LINEAR          GICv2
+    2  0x0001d  GICv2            0xffff00000916bfd8          LINEAR          GICv2
+    3  0x0001e  GICv2            0xffff00000916bfd8     *    LINEAR          GICv2
+    4  0x0001b  GICv2            0xffff00000916bfd8     *    LINEAR          GICv2
+    5  0x0001a  GICv2            0xffff00000916bfd8          LINEAR          GICv2
+[...]
+   96  0x81808  MSI              0x          (null)           RADIX          MSI
+   96+ 0x00063  GICv2m           0xffff8003ee116980           RADIX          GICv2m
+   96+ 0x00063  GICv2            0xffff00000916bfd8          LINEAR          GICv2
+   97  0x08800  MSI              0x          (null)     *     RADIX          MSI
+   97+ 0x00064  GICv2m           0xffff8003ee116980     *     RADIX          GICv2m
+   97+ 0x00064  GICv2            0xffff00000916bfd8     *    LINEAR          GICv2
+
+Here, interrupts 1-5 are only using a single domain, while 96 and 97
+are build out of a stack of three domain, each level performing a
+particular function.