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    Subject[RFC] doc/powerpc: try to explain why the interrupt numbers are off by 16
    Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
    ---
    Documentation/powerpc/dts-bindings/fsl/mpic.txt | 42 +++++++++++++++++++++++
    1 files changed, 42 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/powerpc/dts-bindings/fsl/mpic.txt

    diff --git a/Documentation/powerpc/dts-bindings/fsl/mpic.txt b/Documentation/powerpc/dts-bindings/fsl/mpic.txt
    new file mode 100644
    index 0000000..71e39cf
    --- /dev/null
    +++ b/Documentation/powerpc/dts-bindings/fsl/mpic.txt
    @@ -0,0 +1,42 @@
    +* OpenPIC and its interrupt numbers on Freescale's e500/e600 cores
    +
    +The OpenPIC specification does not specify which interrupt source has to
    +become which interrupt number. This is up to the software implementation
    +of the interrupt controller. The only requirement is that every
    +interrupt source has to have an unique interrupt number / vector number.
    +To accomplish this the current implementation assigns the number zero to
    +the first source, the number one to the second source and so on until
    +all interrupt sources have their unique number.
    +Usually the assigned vector number equals the interrupt number mentioned
    +in the documentation for a given core / CPU. This is however not true
    +for the e500 cores (MPC85XX CPUs) where the documentation distinguishes
    +between internal and external interrupt sources and starts counting at
    +zero for both of them.
    +
    +So what to write for external interrupt source X or internal interrupt
    +source Y into the device tree? Here is an example:
    +
    +The memory map for the interrupt controller in the MPC8544[0] shows,
    +that the first interrupt source starts at 0x5_0000 (PIC Register Address
    +Map-Interrupt Source Configuration Registers). This source becomes the
    +number zero therefore:
    + External interrupt 0 = interrupt number 0
    + External interrupt 1 = interrupt number 1
    + External interrupt 2 = interrupt number 2
    + ...
    +Every interrupt number allocates 0x20 bytes register space. So to get
    +its number it is sufficient to shift the lower 16bits to right by five.
    +So for the external interrupt 10 we have:
    + 0x0140 >> 5 = 10
    +
    +After the external sources, the internal sources follow. The in core I2C
    +controller on the MPC8544 for instance has the internal source number
    +27. Oo obtain its interrupt number we take the lower 16bits of its memory
    +address (0x5_0560) and shift it right:
    + 0x0560 >> 5 = 43
    +
    +Therefore the I2C device node for the MPC8544 CPU has to have the
    +interrupt number 43 specified in the device tree.
    +
    +[0] MPC8544E PowerQUICCTM III, Integrated Host Processor Family Reference Manual
    + MPC8544ERM Rev. 1 10/2007
    --
    1.6.5.3


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