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irqdomain: update documentation 

This updates the IRQdomain documentation a bit, by adding a more
verbose explanation to why we need this, and by adding some
extended documentation of the irq_domain_simple() usecase.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
This commit is contained in:
Linus Walleij 2012-12-01 19:05:16 +01:00 committed by Grant Likely
parent d202b7b970
commit 023bba3638
1 changed files with 33 additions and 1 deletions
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34
Documentation/IRQ-domain.txt
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@ -7,6 +7,21 @@ systems with multiple interrupt controllers the kernel must ensure
that each one gets assigned non-overlapping allocations of Linux that each one gets assigned non-overlapping allocations of Linux
IRQ numbers. IRQ numbers.
The number of interrupt controllers registered as unique irqchips
show a rising tendency: for example subdrivers of different kinds
such as GPIO controllers avoid reimplementing identical callback
mechanisms as the IRQ core system by modelling their interrupt
handlers as irqchips, i.e. in effect cascading interrupt controllers.
Here the interrupt number loose all kind of correspondence to
hardware interrupt numbers: whereas in the past, IRQ numbers could
be chosen so they matched the hardware IRQ line into the root
interrupt controller (i.e. the component actually fireing the
interrupt line to the CPU) nowadays this number is just a number.
For this reason we need a mechanism to separate controller-local
interrupt numbers, called hardware irq's, from Linux IRQ numbers.
The irq_alloc_desc*() and irq_free_desc*() APIs provide allocation of The irq_alloc_desc*() and irq_free_desc*() APIs provide allocation of
irq numbers, but they don't provide any support for reverse mapping of irq numbers, but they don't provide any support for reverse mapping of
the controller-local IRQ (hwirq) number into the Linux IRQ number the controller-local IRQ (hwirq) number into the Linux IRQ number
@ -40,6 +55,10 @@ required hardware setup.
When an interrupt is received, irq_find_mapping() function should When an interrupt is received, irq_find_mapping() function should
be used to find the Linux IRQ number from the hwirq number. be used to find the Linux IRQ number from the hwirq number.
The irq_create_mapping() function must be called *atleast once*
before any call to irq_find_mapping(), lest the descriptor will not
be allocated.
If the driver has the Linux IRQ number or the irq_data pointer, and If the driver has the Linux IRQ number or the irq_data pointer, and
needs to know the associated hwirq number (such as in the irq_chip needs to know the associated hwirq number (such as in the irq_chip
callbacks) then it can be directly obtained from irq_data->hwirq. callbacks) then it can be directly obtained from irq_data->hwirq.
@ -119,4 +138,17 @@ numbers.
Most users of legacy mappings should use irq_domain_add_simple() which Most users of legacy mappings should use irq_domain_add_simple() which
will use a legacy domain only if an IRQ range is supplied by the will use a legacy domain only if an IRQ range is supplied by the
system and will otherwise use a linear domain mapping. system and will otherwise use a linear domain mapping. The semantics
of this call are such that if an IRQ range is specified then
descriptors will be allocated on-the-fly for it, and if no range is
specified it will fall through to irq_domain_add_linear() which meand
*no* irq descriptors will be allocated.
A typical use case for simple domains is where an irqchip provider
is supporting both dynamic and static IRQ assignments.
In order to avoid ending up in a situation where a linear domain is
used and no descriptor gets allocated it is very important to make sure
that the driver using the simple domain call irq_create_mapping()
before any irq_find_mapping() since the latter will actually work
for the static IRQ assignment case.