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

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Message-ID: <F297158F9216834BAB1B3BCC98DBDCCB42D2F3@mailsh.asrmicro.com>
Date:	Mon, 11 Jan 2016 19:15:01 +0800
From:	Zhou Qiao(周侨) <qiaozhou@...micro.com>
To:	"Thomas Gleixner" <tglx@...utronix.de>
Cc:	<jason@...edaemon.net>, <linux-kernel@...r.kernel.org>
Subject: RE: [Questions]: how to extend nr_irqs for arch arm64

> -----Original Message-----
> From: Thomas Gleixner [mailto:tglx@...utronix.de]
> Sent: Monday, January 11, 2016 5:18 PM
> To: Zhou Qiao(周侨)
> 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.
You're right. Actually the virqs is just a next_zero_area picked runtime and it doesn't matter whether the hw_irq is mod or not(or mod by whichever). Thanks a lot for the help.
> 
> Thanks,
> 
> 	tglx