[PATCH 25/25] irq: Document what an IRQ is.

["Eric W. Biederman" <ebiederm@xmission.com>]

From: Eric W. Biederman <ebiederman@lnxi.com>

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
---
 Documentation/IRQ.txt |   22 ++++++++++++++++++++++
 1 files changed, 22 insertions(+), 0 deletions(-)

diff --git a/Documentation/IRQ.txt b/Documentation/IRQ.txt
new file mode 100644
index 0000000..237235d
--- /dev/null
+++ b/Documentation/IRQ.txt
@@ -0,0 +1,22 @@
+What is an IRQ?
+
+An IRQ is an interrupt request from a device.  
+Currently they can come in over a pin, or over a packet.
+Several devices may be connected to the same pin thus
+sharing an IRQ.
+
+An IRQ number is a kernel identifier used to talk about a hardware
+interrupt source.  Typically this is an index into the global irq_desc
+array, but except for what linux/interrupt.h implements the details
+are architecture specific.
+
+An IRQ number is an enumeration of the possible interrupt sources on a
+machine.  Typically what is enumerated is the number of input pins on
+all of the interrupt controller in the system.  In the case of ISA
+what is enumerated are the 16 input pins on the two i8259 interrupt
+controllers.
+
+Architectures can assign additional meaning to the IRQ numbers, and
+are encouraged to in the case  where there is any manual configuration
+of the hardware involved.  The ISA IRQs are a classic example of
+assigning this kind of additional meaning.
-- 
1.4.0.gc07e

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