Re: Generic DT binding for IPIs

From: Qais Yousef
Date: Fri Dec 11 2015 - 05:48:08 EST

On 12/11/2015 12:39 AM, David Gibson wrote:
On Thu, Dec 10, 2015 at 10:20:49AM +0000, Qais Yousef wrote:

The IPIs have two properties that are different from a regular interrupts:

1. An IPI is not only received, it could also be sent.
Any interrupt is sent by the device, received by an interrupt
controller, so this isn't really anything fundamentally different.

No they're not fundamentally different. It's just the way they're created and used.

2. The IPI is dynamic. There's an actual allocation from a pool of
IPIs happening when we ask for one to be reserved.
It's not really clear to me what that means, and why it requires any
particular different information in the device tree.

Maybe it would help to look at the new IPI reservation API?

The difference might be borderline..

Do you have any rough idea on what a possible extension could look like?
Reusing means writing less code, which is always better of course :)

By the way, on MIPS GIC, we can use interrupts property to describe an IPI
the host system will receive. But to send one to the coprocessor, we need to
define an outgoing IPI.
Ah, ok, so is what you're trying to describe here (from the host OS
and CPU point of view) a purely outgoing signal to the coproc?


In this case, the firmware will be hardcoded to send an interrupt to a
specific hwirq, so one can then describe it in DT as a regular interrupt to
the host system. Hardcoding is not ideal and less portable though.
Or is the signal that goes to the coproc then somehow being translated
into a host interrupt? If that's so you should be able to represent
the coproc as an interrupt controller or interrupt nexus.

I'm not sure I understood you completely but no, there's no translation happening. When the IPI is allocated it would be routed
to the coproc. When the host wants to send a signal, it'll use the allocated hwirq value (indirectly via the virq) to write to a register, which will cause an interrupt to be generated at the coproc.


coproc2 {
ipi-refs = <&coproc1 "in">, <&coproc1 "coproc2data">, <&coproc1
This isn't actually parseable. You need a known length of cells after a phandle.

To clarify, what you're saying we can't pass strings, right?
So, I'm not entirely sure what point Rob was making. The above
certainly isn't valid dts syntax - strings can't appear within
the < > construct. But if you make the obvious fix to:
ipi-refs = <&coproc1>, "in", <&coproc1>, "coproc2data";

then it's certainly a parseable property format. It's kind of clunky
mixing integers and strings that way, but it's possible and there are
existing bindings using properties in a similar format.

Ah OK thanks! I think this form would be handy to get the refs even if we end up reusing the interrupts property to allocate an IPI.

So if reusing the interrupts property is the right thing to do, do you (or anyone else) have a rough idea how this should look like?

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