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

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Message-ID: <564EFA74.90606@imgtec.com>
Date:	Fri, 20 Nov 2015 10:48:20 +0000
From:	Qais Yousef <qais.yousef@...tec.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	<linux-kernel@...r.kernel.org>, <jason@...edaemon.net>,
	<marc.zyngier@....com>, <jiang.liu@...ux.intel.com>,
	<ralf@...ux-mips.org>, <linux-mips@...ux-mips.org>
Subject: Re: [PATCH 10/14] irqchip/mips-gic: Add a IPI hierarchy domain

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.

Thoughts?

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.

Thanks,
Qais
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