Date:	Wed, 30 Dec 2015 14:10:58 -0600
From:	Felipe Balbi <balbi@...com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Jason Cooper <jason@...edaemon.net>,
	<linux-kernel@...r.kernel.org>, <linux-omap@...r.kernel.org>,
	Suman Anna <s-anna@...com>, Roger Quadros <rogerq@...com>
Subject: Re: Routable IRQs


Hi,

Thomas Gleixner <tglx@...utronix.de> writes:
> Felipe,
>
> On Wed, 30 Dec 2015, Felipe Balbi wrote:
>> Thomas Gleixner <tglx@...utronix.de> writes:
>> >  - Is there a "mapping" block between PRUSS and the host interrupt controller
>> >    or is this "mapping" block part of PRUSS?
>> 
>> The description in TRM is a bit "poor", but from what I can gather, the
>> mapping is done on an interrupt controller inside the PRUSS. However,
>> Linux is the one who's got the driver for that INTC (well, Linux will be
>> the one with the soft ethernet/uart/whatever IP to talk to). All of its
>> (INTC's) registers are memory mapped to the ARM side.
>
> Ok. And the INTC registers include the "mapping" configuration, right?

right. A bunch of 32 bit registers each with several 4 bit fields (one
for each of the 64 events) where we write the physical IRQ number.

>> >  - We all know how well shared interrupts work. Is there a point of supporting
>> >    64 interrupts when you only have 10 irq lines available?
>> 
>> I'm looking at these 64 events more like MSI kind of events. It's just
>
> Well, that's fine to look at them this way, but they will end up
> shared no matter what.

sure :-)

>> that the events themselves can be routed to any of the 10 available HW
>> IRQ lines.
>> 
>> >  - I assume that the PRUSS interrupt mapping is more or less a question of the
>> >    firmware implementation. So you either have a fixed association in the
>> >    firmware which is reflected in the DT description of the IP block or you
>> >    need an interface to tell the PRUSS firmware which event it should map to
>> >    which irq line. Is there actually a value in doing the latter?
>> 
>> right, I'd say the mapping is pretty static. Unless Suman has some extra
>> information which I don't. I guess the question was really to see if
>> there was an easy way for doing this so we don't have to mess with DTS
>> for every other FW and their neighbor.
>
> Well, you will need information about every other firmware simply because you
> need to know which events the firmware is actually using and what the purpose
> of the particular event is.
>
> Assume you have a simple uart with 3 events (RX, TX, status). So how will the
> firmware tell you which event is which? You have a few options:
>
>  1) DT + fixed mapping scheme: 
>
>     Describe the PRUSS event number in DT and have a fixed mapping scheme like
>     the one you mentioned evt0 -> irq0 .....
>
>  2) DT + DT mapping scheme
>
>     Describe the PRUSS event number in DT and describe the mapping scheme in
>     DT as well
>
>  3) DT + dynamic mapping scheme
>
>     Describe the PRUSS event number in DT and let your interrupt controller
>     associate the irq number dynamically. That's kind of similar to MSI with
>     the exception that it needs to support shared interrupts.
>
>  4) Fully dynamic association
>
>     Have a query interface to the firmware which tells you which event it uses
>     for which particular purpose (RX, TX ...) and then establish a dynamic
>     mapping to one of the interrupts.
>
> Not sure which level of complexity you want :)

I guess only 1, 2 are anything worth considering, most likely. 4 would
just be too much headache :-p

3 might be doable too, though a bit more complex. Suman (who has been
working on this for much longer than I have) might have some extra info
to add, but he's on vacations for now. Hopefully, he'll add to this
thread once he's back.

cheers

-- 
balbi

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