[RFC][PATCH] Document what in IRQ is.

[ebiederm@xmission.com (Eric W. Biederman)]

Andi Kleen <ak@suse.de> writes:

> P.S.: There seems to be a lot of confusion about all this.
> Maybe it would make sense to do a write up defining all the terms
> and stick it into Documentation/* ? 

How does this look?

I am pretty horrible when it comes to Documentation,
but this seems to be the essence of what I was saying earlier.

Eric


diff --git a/Documentation/IRQ.txt b/Documentation/IRQ.txt
new file mode 100644
index 0000000..5340369
--- /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.
+IRQs at the source can be shared.
+
+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 to the pair of 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 IRQ case on x86 where anyone who
+has been around a while can tell you how the first 16 IRQs map to the
+input pins on a pair of i8259s is the classic example.
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