linux-kernel - Re: [PATCH 1/3] misc: Add crossbar driver

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Date:	Wed, 21 Aug 2013 23:10:14 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Santosh Shilimkar <santosh.shilimkar@...com>
Cc:	Sricharan R <r.sricharan@...com>, Tony Lindgren <tony@...mide.com>,
	Nishanth Menon <nm@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux-OMAP <linux-omap@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Rajendra Nayak <rnayak@...com>, Felipe Balbi <balbi@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Grant Likely <grant.likely@...retlab.ca>
Subject: Re: [PATCH 1/3] misc: Add crossbar driver

On Thu, Aug 15, 2013 at 11:14 PM, Santosh Shilimkar
<santosh.shilimkar@...com> wrote:
> On Thursday 15 August 2013 04:51 PM, Linus Walleij wrote:
(...)
>> Sorry I don't understand what thread that is... can you point me there?
>> My previous statement on this issue what this:
>> http://marc.info/?l=linux-kernel&m=137442541628641&w=2
(...)
>>> I don't see how you can make this happen with an irqchip
>>> infrastructure.
>>
>> I think my post above describes this.
>>
> Sorry for being dumb but I don't think cascaded irqchip examples
> like GPIO and cross-bars are same. If you take an example of
> GPIO irqchip, it always have a physical connection even if it
> is 1 IRQ line for (32 logical/sparse IRQs). That goes with
> other MFD examples too.
>
> So may be I am still missing something in your proposal.

Why does it matter if it is a GPIO or MFD or whatever?
The point is that the IRQ line passes thru something else,
and this we model as an irqdomain.

Anyway here is a silicon cascaded IRQ chip:
arch/arm/mach-versatile/core.c
See versatile_init_irq():

__vic_init(VA_VIC_BASE, IRQ_VIC_START, ~0, 0, np);
(...)
fpga_irq_init(VA_SIC_BASE, "SIC", IRQ_SIC_START,
                IRQ_VICSOURCE31, PIC_VALID, np);

The VIC in the versatile has the SIC cascaded from one of
its IRQ lines. Both the VIC and SIC (fpga IRQ) are
using irqdomains so the SIC spawns a child irqdomain
from IRQ 31 (last IRQ) of the VIC.

The difference with a crossbar is that it can software-config
which IRQ goes where, and does not have a callback
to clear interrupts or anything like that, it just passes them
thru. But it is best modeled as an irqdomain IMO.

Yours,
Linus Walleij
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