From mboxrd@z Thu Jan  1 00:00:00 1970
From: ludovic.desroches@atmel.com (ludovic.desroches)
Date: Mon, 18 Jun 2012 14:10:24 +0200
Subject: How to use linear mapping without DT?
In-Reply-To: <20120616023507.GC29663@S2101-09.ap.freescale.net>
References: <4FD9B4DC.80706@atmel.com>
 <20120615160155.GN31565@S2101-09.ap.freescale.net>
 <4FDB639D.4060103@atmel.com>
 <20120616023507.GC29663@S2101-09.ap.freescale.net>
Message-ID: <4FDF1AB0.5@atmel.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

Le 06/16/2012 04:35 AM, Shawn Guo a ?crit :
> On Fri, Jun 15, 2012 at 06:32:29PM +0200, ludovic.desroches wrote:
>> I've seen your patch series and I am wondering what is the
>> difference between these two solutions:
>> - not using nr_irqs: you will have NR_IRQS_LEGACY irqdescs allocated
>> and you will allocate the irqdescs for your controller just after by
>> calling irq_alloc_descs so you will have to manage the
>> NR_IRQS_LEGACY offset.
>> - using nr_irqs: you will have all your irqdescs allocated and won't
>> have to manage the NR_IRQS_LEGACY offset.
>>
> First of all, NR_IRQS_LEGACY shifting is a good thing we are actually
> looking for because we want to reserve virq number 0 for no-interrupt
> case. [1]

I don't think it's an issue for legacy domain since virq number 0 is 
ignored.

>
> Secondly, with .nr_irqs approach, you will have to somehow maintain
> irq_base for each irqchip, which is needed when adding legacy domain.
> Calling irq_alloc_descs will just return you the irq_base.
>
> Lastly, irq_alloc_descs approach can save irqdesc than .nr_irq in some
> cases, where not all the irqchips get instantiated for every single
> boot.  That's said, .nr_irqs has to be set the possible maximum numbers
> for the machine, while some irqchips, e.g. peripheral/device inside
> ones, are not always instantiated.  irq_alloc_descs approach will save
> irqdescs for such cases.

Of course it makes sense.

Thanks for these explanations. I'll correct my patches to use the 
NR_IRQS_LEGACY shifting instead of using nr_irqs.

Regards

Ludovic