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authorEric W. Biederman <ebiederman@xmission.com>2006-06-29 02:25:02 -0700
committerLinus Torvalds <torvalds@g5.osdl.org>2006-06-29 10:26:25 -0700
commitf702d7013c7470284843a6370aaa53b8b75c5a40 (patch)
tree1989bc89230b8319b3e2007b6e6238cc2dcec415 /Documentation/IRQ.txt
parent98bb244b685eb2a297aa60fa2e5c0631f95828e1 (diff)
downloadleg-kernel-f702d7013c7470284843a6370aaa53b8b75c5a40.tar.gz
[PATCH] genirq: irq: document what an IRQ is
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Rajesh Shah <rajesh.shah@intel.com> Cc: Andi Kleen <ak@muc.de> Cc: "Protasevich, Natalie" <Natalie.Protasevich@UNISYS.com> Cc: "Luck, Tony" <tony.luck@intel.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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+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.