Re: [PATCH] powerpc: document new interrupt-array property

[Segher Boessenkool <segher@kernel.crashing.org>]

>> It would be nicer to keep the "interrupts" property and
>> add a property "interrupt-parents" with the same number
>> of entries, that encodes the same as "interrupt-parent"
>> but per interrupt.  You keep more in line with the
>> "normal" stuff and I suspect it's less code to parse as
>> well.
>
> There are pro and cons to this approach. I did think about it 
> initially.
> The main cons is that "interrupts" becomes harder to parse as you ahve
> to walk interrupt-parents at the same time to get the #interrupt-cells
> of each entry.

Yeah, you have to do some things inside of the interrupt
parsing loop, that you normally can move outside of the
loop.  That's true in the "interrupt-array" case too, you
just get the interrupt parent from somewhere else.  It's
in the nature of this extension.

> Also, it adds more potential for stupid breakage (how
> shluld the parser react if interrupt-parents has less entries than
> interrupts ?)

Just fail loudly.  Just like it should do if it notes
something else nonsensical (a non-phandle in the
"interrupt-array" case, for example, or running out
of encoded integers while parsing).

> The pro, which is quite important too, is that it's a common assuption
> that you have interrupts when you have an "interrupts" property, 
> period.

Yes, it would be a lot closer to the generic case, only add
to the interrupt mapping binding, not change anything in the
base OF spec.

> In fact, it would indeed fit a bit better in the current parser.

Yeah I think so too.

>> Wrong place to document this?  It's true for all interrupt
>> specifiers.
>
> Might be worth giving a crazy example where the 2 interrupt specifiers
> have different size.

I'd rather not.  Anyway, this was in reply to:

>> +Note: the number of cells needed to represent the
>> +interrupt-specifier is determined by the #interrupt-cells
>> +property of the interrupt parent.

Which is a general statement about interrupt specifiers.
Nowhere does the "booting without OF" doc define interrupt
specifiers before it uses them; do that, move this comment
there, and put a comment saying "each entry in "interrupts"
can potentially have a different # of interrupt cells, you
have to look at the corresponding interrupt parent to find
out" in its place?


Segher