Date: Fri, 1 Aug 2008 10:51:20 -0500
From: Cliff Wickman <cpw@sgi.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Alan Mayer <ajm@sgi.com>, jeremy@goop.org, rusty@rustcorp.com.au,
       suresh.b.siddha@intel.com, mingo@elte.hu, torvalds@linux-foundation.org,
       linux-kernel@vger.kernel.org, Dean Nelson <dcn@sgi.com>
Subject: Re: [PATCH] x86_64:  Dynamically allocate arch specific system
	vectors
Message-ID: <20080801155120.GA21706@sgi.com>
References: <48922D16.1080704@sgi.com> <m13alp1z8w.fsf@frodo.ebiederm.org>
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On Thu, Jul 31, 2008 at 03:10:23PM -0700, Eric W. Biederman wrote:
> Alan Mayer <ajm@sgi.com> writes:
> 
> > Subject: [PATCH] x86_64:  Dynamically allocate arch specific system vectors
> >
> > From: Alan Mayer <ajm@sgi.com>
> >
> > On some systems (e. g., UV) it is necessary to use an interrupt vector
> > as a "system" vector, that is, it is generated by system hardware, not an
> > IO device.  This patch dynamically allocates them from the pool of interrupt
> > vectors below the fixed system vectors.  This may include stealing some from
> > the device interrupt vector pool, so they are allocated dynamically so that
> > other archs don't have to pay the price.  In UV, examples of these hardware
> > and software systems that need dynamically allocated vectors are the GRU,
> > the BAU, and XPM/XPC.
> 
> Could you please explain words why the vector allocator does not work
> for you?
> 
> This code at first glance looks a lot like duplicating the vector allocator.
> 
> Eric

I assume you mean create_irq() and destroy_irq().
They are close to what we need.

However for any given system use:
- we need to request a high priority vector for some irq's, rather than
  one randomly allocated as per __assign_irq_vector().
- we want the irq/vector to be targeted to all cpu's (as specified in a
  mask,
  and can include currently offline cpu's) rather than a single cpu. 

I suppose those abilities could be added to create_irq(), but we didn't
want to intrude into that interface.

A smaller consideration is simplicity of use.  We want any such user to
use
the generic do_IRQ() flow (not alloc_intr_gate()).  But make it easy to
set
up the irq/vector, irq_chip and irq_desc without getting intimate with
the
details.
I suppose some other wrapper for an enhanced create_irq() could be done.

We are going to need such irq/vector pairs for a couple of UV drivers
(drivers/misc/sgi-gru/ and sgi-xp/).  And would prefer it for the UV TLB
shootdown (x86/kernel/tlb.uv.c) rather than using alloc_intr_gate().


-Cliff
-- 
Cliff Wickman
Silicon Graphics, Inc.
cpw@sgi.com
(651) 683-3824
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