linux-kernel - RE: [Questions]: how to extend nr

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Date:	Mon, 11 Jan 2016 10:17:44 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Zhou Qiao(周侨) <qiaozhou@...micro.com>
cc:	jason@...edaemon.net, linux-kernel@...r.kernel.org
Subject: RE: [Questions]: how to extend nr_irqs for arch arm64

On Mon, 11 Jan 2016, Zhou Qiao(周侨) wrote:
> > From: Thomas Gleixner [mailto:tglx@...utronix.de]
> > On Mon, 11 Jan 2016, Zhou Qiao(周侨) wrote:
> > > I met an issue here. The nr_irqs is extended when the start_irq + cnt
> > > > nr_irqs. So when nr_irqs is not extended(still 64), a device with a
> > > large
> > > hr_irq(70) will get a wrong virq. since it will be mod by nr_irqs
> > > before
> > 
> > No, it won't get a wrong virq. It will get a virq and it does not
> > matter at all whether the hardware irq number is 10, 70, 500 or 5000.
> > What's your problem?
> 
> Let's assume the hw_irq is 70, and after the mod operation, it will call
> irq_alloc_descs_from with parameter from = 6, instead of 70. So the newly
> allocated descriptor is mapped to hw_irq = 6, not 70. Later when driver
> registers irq with irq = 70, it will fail since there is no irq descriptor
> for irq = 70.

Rightfully so. The driver cannot assume irq=70. That's what the device tree
mapping is for. It will tell the driver that it has to request virq 6. There
is no 1:1 mapping between hardware irqs and virqs.

Thanks,

	tglx