Re: [PATCH 10/14] irqchip/mips-gic: Add a IPI hierarchy domain

From: Qais Yousef
Date: Fri Nov 20 2015 - 05:48:32 EST

Hi Thomas,

On 11/16/2015 05:17 PM, Thomas Gleixner wrote:
1) IPI as per_cpu interrupts

Single hwirq represented by a single irq descriptor

2) IPI with consecutive mapping space

No extra mapping from virq base to target cpu required as its just
linear. Everything can be handled via the base virq.

I think I am seeing a major issue with this approach.

Take the case where we reserve an IPI with ipi_mask that has cpu 5 and 6 set only. When allocating a per_cpu or consectuve mapping, we will require 2 consecutive virqs and hwirqs. But since the cpu location is not starting from 0, we can't use the cpu as an offset anymore.

So when a user wants to send an IPI to cpu 6 only, the code can't easily tell what's the correct offset from base virq or hwirq to use.

Same applies when doing the reverse mapping.

In other words, the ipi_mask won't always necessarily be linear to facilitate the 1:1 mapping that this approach assumes.

It is a solvable problem, but I think we're losing the elegance that promoted going into this direction and I think sticking to using struct ipi_mapping (with some enhancements to how it's exposed an integrated by/into generic code) is a better approach.


I still don't have a working implementation otherwise I would have sent my patches, but I thought I'd raise this up before I spend more time on it unnecessarily.

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