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

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Date:	Thu, 15 Aug 2013 22:51:50 +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 10:26 PM, Santosh Shilimkar
<santosh.shilimkar@...com> wrote:
> On Thursday 15 August 2013 04:01 PM, Linus Walleij wrote:
>> On Tue, Aug 13, 2013 at 11:56 AM, Sricharan R <r.sricharan@...com> wrote:
>>
>>>  Initially irqchip was discussed, but we also have a DMA crossbar
>>>  to map the dma-requests. Since both irq/dma crossbars should be handled,
>>>  pinctrl was suggested as the appropriate place to handle this.
>>
>> I think it is better to use irqchip.
>>
> Did you happen to read the thread why irqchip is in-appropriate
> for such an IP.

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

> As I said earlier, an IRQ-chip always need a
> real IRQ link (even for the chained one) to the primary irqchip.
>
> This IP is just dummy IP makes the connections for the primary
> irqchip(read GIC). And its use only limited to make the
> connection between the peripheral IRQ event to the GIC IRQ line.
>
> I don't see how you can make this happen with an irqchip
> infrastructure.

I think my post above describes this.

>> I don't see any way to really abstract this pretty simple crossbar
>> for reuse across subsystems.
>>
> This exactly the reason, i am against idea of over-engineering the
> simple IP whose only job is to make the physical wire connection
> in software where as this is generally done in RTL by default on
> most of the SOCs.

Well, it was made accessible by software, and if someone has a
usecase that requires this do be done dynamically, i.e. not just
being set up by firmware and never touched, and that use case
is valid, then I guess we need to do something...

I think it was mentioned in the thread that there is really such
a usecase?

Yours,
Linus Walleij
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