linux-kernel - Re: [PATCH] pci: change msi-x vector to 32bit

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Message-ID: <20080816201023.GA271@plap4-2.local>
Date:	Sat, 16 Aug 2008 13:10:23 -0700
From:	Andrew Vasquez <andrew.vasquez@...gic.com>
To:	Yinghai Lu <yhlu.kernel@...il.com>
Cc:	James Bottomley <James.Bottomley@...senpartnership.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pci: change msi-x vector to 32bit

On Sat, 16 Aug 2008, Yinghai Lu wrote:

> On Sat, Aug 16, 2008 at 9:13 AM, James Bottomley
> <James.Bottomley@...senpartnership.com> wrote:
> > On Sat, 2008-08-16 at 16:39 +0100, Alan Cox wrote:
> >> > Where exactly is this code in the kernel?  Most arches assume the irq is
> >> > an index to a compact table bounded by NR_IRQS, so something like this
> >> > would violate that assumption.
> >>
> >> Yes, which is no bad thing for some platforms. There are some driver
> >> assumptions like that but those have also been stomped.
> >
> > I'm not saying we couldn't do this, or even that we shouldn't; I'm just
> > asking why would we want to?
> >
> > All arches currently seem to have show_interrupts() which loop over
> > 0..NR_IRQS where the interrupt is printed as %d.  In this encoded scheme
> > they would show up with rather nastily large numbers that have no
> > visible meaning unless we switch to hex for displaying them.
> >
> > What I'm really saying is that irq as the interrupt number is really the
> > *user's* handle for the interrupt not the machine's, so it needs to be
> > something the user is comfortable with.  We could overcome this
> > objection by encoding the number to something meaningful for the
> > user ... I'm just asking if there's any benefit to doing this?
> >
> the code is tip/irq/sparseirq or tip/master
> 
> story:
> 1. for x86_64: first we have NR_IRQS = NR_CPUS * NR_VECTORS, because
> it already supports per_cpu vector
> 2. SGI want MAX_SMP support: NR_CPUS=4096, so everything is broken.
> 3. Mike spent some time to make every array [NR_CPUS]  to per_cpu
> define as possible.
> 4. Mike or someone else reduce NR_IRQS to 224, because NR=256*4096,
> will make kstat_irqs[NR_CPUS][NR_VECTORS*NR_VECTORS] too big, and it
> could be complied.
> 5. IBM guys report their one server is broken, that system GSI > 256,
> so some irq can not work.
> 6. Yinghai tried one patch change NR_IRQS=32*NR_CPUS., but sgi said it
> still broke their system.  --- for 2.6.27
> 7. Eric provide one patch NR_IRQS = min(32*NR_CPUS, NR_VECTORS *
> MAX_IO_APICS) --- for 2.6.27
> 8. For 2.6.28 later, Yinghai add code dyn_array, and probe nr_irqs, so
> NR_IRQS related will be dynamically allocated after nr_irqs is probed.
> 9. Eric said using dyn_array still waste ram, because a lot of
> irq_desc is not used. when MSI-X is involved, some card could use 256
> vectors or 4096 in theory.
> 10. Eric said he had one dyn irq_desc, with 90% done. but didn't have
> time to work it out left 10%
> 11. Yinghai add sparese_irq support. those array will be increased by
> 32, and be claimed one by one.
> 12. according to Eric, we could have irq spread out [0, -1U), irq =
> bus/dev/fn + entry_of_msix
> 13. with sparseirq, /proc/interrupts will have irq_number in hex.
> 
> but msix current cached irq number, and it only use 16bit to store
> unsigned int irq., and later cards will call request_irq with
> truncated irq_number...card will fallback to MSI or INTa
> 
> only two places need to be changed about that.
> 
> BTW, any reason qlogic card need to cache that irq number second times?

So that the driver can release the two request_irq() allocated
handlers during tear-down (via qla24xx_disable_msix()->free_irq()).
Beyond caching (vector/irq) what's returned during pci_enable_msix(),
is there some other mechanism a driver can use to get the IRQ number?
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