[PATCH][Natty] PCI: allow matching of prefetchable resources to non-prefetchable windows

[Steve Conklin]

Applied to Lucid

On Mon, 2011-05-23 at 16:36 -0500, Seth Forshee wrote:
> From: Linus Torvalds <torvalds at linux-foundation.org>
> 
> BugLink: http://bugs.launchpad.net/bugs/424142
> 
> I'm not entirely sure it needs to go into 32, but it's probably the right
> thing to do. Another way of explaining the patch is:
> 
>  - we currently pick the _first_ exactly matching bus resource entry, but
>    the _last_ inexactly matching one. Normally first/last shouldn't
>    matter, but bus resource entries aren't actually all created equal: in
>    a transparent bus, the last resources will be the parent resources,
>    which we should generally try to avoid unless we have no choice. So
>    "first matching" is the thing we should always aim for.
> 
>  - the patch is a bit bigger than it needs to be, because I simplified the
>    logic at the same time. It used to be a fairly incomprehensible
> 
> 	if ((res->flags & IORESOURCE_PREFETCH) && !(r->flags & IORESOURCE_PREFETCH))
> 		best = r;       /* Approximating prefetchable by non-prefetchable */
> 
>    and technically, all the patch did was to make that complex choice be
>    even more complex (it basically added a "&& !best" to say that if we
>    already gound a non-prefetchable window for the prefetchable resource,
>    then we won't override an earlier one with that later one: remember
>    "first matching").
> 
>  - So instead of that complex one with three separate conditionals in one,
>    I split it up a bit, and am taking advantage of the fact that we
>    already handled the exact case, so if 'res->flags' has the PREFETCH
>    bit, then we already know that 'r->flags' will _not_ have it. So the
>    simplified code drops the redundant test, and does the new '!best' test
>    separately. It also uses 'continue' as a way to ignore the bus
>    resource we know doesn't work (ie a prefetchable bus resource is _not_
>    acceptable for anything but an exact match), so it turns into:
> 
> 	/* We can't insert a non-prefetch resource inside a prefetchable parent .. */
> 	if (r->flags & IORESOURCE_PREFETCH)
> 		continue;
> 	/* .. but we can put a prefetchable resource inside a non-prefetchable one */
> 	if (!best)
> 		best = r;
> 
>    instead. With the comments, it's now six lines instead of two, but it's
>    conceptually simpler, and I _could_ have written it as two lines:
> 
> 	if ((res->flags & IORESOURCE_PREFETCH) && !best)
> 		best = r;	/* Approximating prefetchable by non-prefetchable */
> 
>    but I thought that was too damn subtle.
> 
> Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
> Signed-off-by: Jesse Barnes <jbarnes at virtuousgeek.org>
> (cherry picked from commit 8c8def26bfaa704db67d515da3eb92cf26067548)
> 
> Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
> ---
>  drivers/pci/pci.c |    8 ++++++--
>  1 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 812d4ac..0d3326d 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -373,8 +373,12 @@ pci_find_parent_resource(const struct pci_dev *dev, struct resource *res)
>  			continue;	/* Wrong type */
>  		if (!((res->flags ^ r->flags) & IORESOURCE_PREFETCH))
>  			return r;	/* Exact match */
> -		if ((res->flags & IORESOURCE_PREFETCH) && !(r->flags & IORESOURCE_PREFETCH))
> -			best = r;	/* Approximating prefetchable by non-prefetchable */
> +		/* We can't insert a non-prefetch resource inside a prefetchable parent .. */
> +		if (r->flags & IORESOURCE_PREFETCH)
> +			continue;
> +		/* .. but we can put a prefetchable resource inside a non-prefetchable one */
> +		if (!best)
> +			best = r;
>  	}
>  	return best;
>  }
> -- 
> 1.7.0.4
> 
> 

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