linux-kernel - Re: [PATCH RFC 4/4] irq: allow a per-allocation upper limit when allocating irqs

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Date:	Thu, 26 May 2011 21:38:15 -0600
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Milton Miller <miltonm@....com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 4/4] irq: allow a per-allocation upper limit when
 allocating irqs

On Wed, May 25, 2011 at 01:34:18AM -0500, Milton Miller wrote:
> Allow the option to specify an upper limit to the irq numbers for
> each allocation.  The limit is non-inclusive, and 0 means no limit
> needed by caller.
> 
> Some irq chips can support a relative large and arbitrary range,
> but not infinite.  For example, they may use the linux irq number
> as the msi desciptor data, which for msi is 16 bits.
> 
> Since e7bcecb7b1 (genirq: Make nr_irqs runtime expandable), checking
> NR_IRQS or even nr_irqs is not sufficient to enforce such requirements
> when sparse irqs are configured.
> 
> Based on an irc discussion, make the limit per call instead of an
> arch callback or global setting.
> 
> If the requested count is above the limit, return -ENOMEM as if
> nr_irqs could not be expanded, assuming the limit was specified at
> a different layer than the count.
> 
> This code does not try to keep the prior semantics of irq_alloc_descs
> when irq was non-negative but not equal to from.  I believe it was an
> implementation artifact instead of a designed feature that one could
> specify search starting from 4 and fail if the allocated irq was not
> exactly 6, confirming that the intervening irqs were reserved.
> 
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Milton Miller <miltonm@....com>

Looks good to me/

g.

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