Re: [PATCH 01/10] irqchip: irq-mips-gic: export gic_send_ipi

From: Qais Yousef
Date: Wed Sep 02 2015 - 06:49:49 EST

On 09/02/2015 10:55 AM, Marc Zyngier wrote:
On 02/09/15 10:33, Qais Yousef wrote:
On 08/28/2015 03:22 PM, Thomas Gleixner wrote:
On Fri, 28 Aug 2015, Qais Yousef wrote:
Thanks a lot for the detailed explanation. I wasn't looking for a quick and
dirty solution but my view of the problem is much simpler than yours so my
idea of a solution would look quick and dirty. I have a better appreciation of
the problem now and a way to approach it :-)

From DT point of view are we OK with this form then

coprocessor {
interrupt-source = <&intc INT_SPEC COP_HWAFFINITY>;
interrupt-sink = <&intc INT_SPEC CPU_HWAFFINITY>;

and if the root controller sends normal IPI as it sends normal device
interrupts then interrupt-sink can be a standard interrupts property (like in
my case)

coprocessor {
interrupt-source = <&intc INT_SPEC COP_HWAFFINITY>;
interrupts = <INT_SPEC>;

Does this look right to you? Is there something else that needs to be covered
I'm not an DT wizard. I leave that to the DT experts.
Hi Marc Zyngier, Mark Rutland,

Any comments about the DT binding for the IPIs?

To recap, the proposal which is based on Marc Zyngier's is to use
interrupt-source to represent an IPI from Linux CPU to a coprocessor and
interrupt-sink to receive an IPI from coprocessor to Linux CPU.
Hopefully the description above is self explanatory. Please let me know
if you need more info. Thomas covered the routing, synthesising, and
requesting parts in the core code. The remaining (high level) issue is
how to describe the IPIs in DT.
I'm definitely *not* a DT expert! ;-) My initial binding proposal was
only for wired interrupts, not for IPIs. There is definitely some common
aspects, except for one part:

Who decides on the IPI number? So far, we've avoided encoding IPI
numbers in the DT just like we don't encode MSIs, because they are
programmable things. My feeling is that we shouldn't put the IPI number
in the DT because the rest of the kernel uses them as well and could
decide to use this particular IPI number for its own use: *clash*.

I think this is covered in Thomas proposal to reserve IPIs. His thoughts is to use a separate irq-domain for IPIs and use irq_reserve_ipi() and irq_destroy_ipi() to get and release IPIs.

The way I see it would be to have a pool of IPI numbers that the kernel
requests for its own use first, leaving whatever remains to drivers.

That's what Thomas thinks too and he covered this by using irq_reserve_ipi() and irq_destroy_ipi().

It's worth noting in the light of this that INT_SPEC should be optional since for hardware similar to mine there's not much to tell the controller if it's all dynamic except where we want the IPI to be routed to - the INT_SPEC is implicitly defined by the notion it's an IPI.


Mark (as *you* are the expert ;-), what do you think?


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