linux-kernel - Re: Multiple MSI

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Message-Id: <1215060119.21182.77.camel@pasglop>
Date:	Thu, 03 Jul 2008 14:41:59 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Matthew Wilcox <matthew@....cx>
Cc:	linux-pci@...r.kernel.org,
	Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Miller <davem@...emloft.net>,
	Dan Williams <dan.j.williams@...el.com>,
	Martine.Silbermann@...com, linux-kernel@...r.kernel.org,
	Michael Ellerman <michaele@....ibm.com>
Subject: Re: Multiple MSI

On Wed, 2008-07-02 at 21:59 -0600, Matthew Wilcox wrote:
> 
> 
> This is true and worth considering carefully.  Are IRQ numbers a scarce
> resource on PowerPC?  They are considerably less scarce than interrupt
> vectors are on x86-64.  How hard is it to make IRQ numbers an abundent
> resource?  Is it simply a question of increasing NR_IRQS?

Yes, indeed, they aren't really scarce... actually less than the
underlying HW vectors in most cases, so it isn't a big issue to add some
kind of constraint to the allocator.

> This cost should be traded off against the cost of allocating something
> like the msix_entry array in each driver that wants to use multiple MSIs,
> passing that array around, using it properly, etc.
> 
> It would make some sense to pass nr_irqs all the way down to arch code
> and let arch code take care of reserving the block of vectors (aligned
> appropriately).  That would conserve IRQ numbers, though not vectors.
> I think we have to consider excess vectors reserved.  If we don't, we
> could get into the situation where a device uses more interrupts than
> the driver thinks it will and problems ensue.

Ok, so I lift my objection there in the sense that allocating a linear
array of virtual numbers shouldn't be a problem (somebody remind me to
make NR_IRQS a config option one of these days on ppc, or help with just
getting rid of irq_desc array alltogether :-)

However, do you want to still keep the fact that they are power-of-2
aligned up to the API or can I just do a linear block allocation for
virtual number sand require drivers to do the appropriate
addition/subtraction to get the N'th one ? I will need to allocate
appropriately aligned HW numbers but that's done via different
mechanisms (and in some case not even under full linux control, ie,
hypervisor/firmware does it on pSeries).

> By the way, would people be interested in changing the MSI-X API to get
> rid of the msix_entry array?  If allocating consecutive IRQs isn't a
> problem, then we could switch the MSI-X code to use consecutive IRQs.

It would make a lot of code simpler...

Ben.


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