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Date:	Sun, 03 Oct 2010 17:49:51 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	Russell King <linux@....linux.org.uk>,
	David Woodhouse <dwmw2@...radead.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Yinghai Lu <yinghai@...nel.org>,
	Grant Likely <grant.likely@...retlab.ca>
Subject: Re: [patch 00/47] Sparse irq rework

Thomas Gleixner <tglx@...utronix.de> writes:

> On Sun, 3 Oct 2010, Eric W. Biederman wrote:
>> Thomas Gleixner <tglx@...utronix.de> writes:
>> > Rationale:
>> > ----------
>> >
>> > The current sparse_irq allocator has several short comings due to
>> > failures in the design or the lack of it:
>> >
>> >  - Requires iteration over the number of active irqs to find a free slot
>> >    Some architectures have grown their own workarounds for this.
>> >
>> >  - Freeing of irq descriptors is not possible
>> >
>> >  - Racy between create_irq_nr and destroy_irq plugged by horrible
>> >    callbacks
>> >
>> >  - Migration of active irq descriptors is not possible
>> 
>> I believe you have distored the design when aiming for migration
>> of active irq descriptors (which you have not even implemented yet).
>> 
>> How do you plan to remove the radix tree lookup from the irq
>> handling path?
>
> Not at all and it's not even even a requirement to remove the lookup
> for implementing live migration.

It sounds like it is a requirement to *keep* the lookup for supporting
live migration.  *Keeping* the lookup I see as a serious problem.  If we
do this right the only users of the radix tree will be drivers using the
functions in interrupt.h.

>> Those files provide the genirq irq chip implementation especially
>> drivers/pci/msi.c.  Of course they will do what every other irq_chip
>> implementation does to get access to data.  There is an unpleasant
>> difference between which generic irq data field htirq.c uses and msi.c
>> which may be worth cleaning up.  But otherwise I don't see any
>> fundamental problems.
>
> The fundamental problem I hit, was the hack which handed down irq_desc
> to avoid the lookup. If it had been msi_desc in the first place, then
> I would not even need to touch the msi code to cleanup x86.

Just because you intend to rename the irq_desc irq_data...

It isn't a hack for an irq method to look at irq_desc.  At least not
until your irq_data changes go through.  This has nothing to do with
how x86 is structured and everything to do with your irq_data
``cleanup'' which appears to be mostly about code churn, for very little
apparent benefit.

In the current state of the kernel I find it very hard to swallow that
having a genirq client using irq_desc (which is the only way to
implement somethings) is a hack.

Eric
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