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Date: Wed, 27 Aug 2014 16:18:54 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Mike Travis <travis@sgi.com>
Cc: mingo@redhat.com, tglx@linutronix.de, hpa@zytor.com, msalter@redhat.com,
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        mgorman@suse.de, linux-kernel@vger.kernel.org, x86@kernel.org,
        linux-mm@kvack.org, Alex Thorlton <athorlton@sgi.com>
Subject: Re: [PATCH 1/2] x86: Optimize resource lookups for ioremap
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On Wed, 27 Aug 2014 16:09:09 -0700 Mike Travis <travis@sgi.com> wrote:

> 
> >>
> >> ...
> >>
> >> --- linux.orig/kernel/resource.c
> >> +++ linux/kernel/resource.c
> >> @@ -494,6 +494,43 @@ int __weak page_is_ram(unsigned long pfn
> >>  }
> >>  EXPORT_SYMBOL_GPL(page_is_ram);
> >>  
> >> +/*
> >> + * Search for a resouce entry that fully contains the specified region.
> >> + * If found, return 1 if it is RAM, 0 if not.
> >> + * If not found, or region is not fully contained, return -1
> >> + *
> >> + * Used by the ioremap functions to insure user not remapping RAM and is as
> >> + * vast speed up over walking through the resource table page by page.
> >> + */
> >> +int __weak region_is_ram(resource_size_t start, unsigned long size)
> >> +{
> >> +	struct resource *p;
> >> +	resource_size_t end = start + size - 1;
> >> +	int flags = IORESOURCE_MEM | IORESOURCE_BUSY;
> >> +	const char *name = "System RAM";
> >> +	int ret = -1;
> >> +
> >> +	read_lock(&resource_lock);
> >> +	for (p = iomem_resource.child; p ; p = p->sibling) {
> >> +		if (end < p->start)
> >> +			continue;
> >> +
> >> +		if (p->start <= start && end <= p->end) {
> >> +			/* resource fully contains region */
> >> +			if ((p->flags != flags) || strcmp(p->name, name))
> >> +				ret = 0;
> >> +			else
> >> +				ret = 1;
> >> +			break;
> >> +		}
> >> +		if (p->end < start)
> >> +			break;	/* not found */
> >> +	}
> >> +	read_unlock(&resource_lock);
> >> +	return ret;
> >> +}
> >> +EXPORT_SYMBOL_GPL(region_is_ram);
> > 
> > Exporting a __weak symbol is strange.  I guess it works, but neither
> > the __weak nor the export are actually needed?
> > 
> 
> I mainly used 'weak' and export because that was what the page_is_ram
> function was using.  Most likely this won't be used anywhere else but
> I wasn't sure.  I can certainly remove the weak and export, at least
> until it's actually needed?

Several architectures implement custom page_is_ram(), so they need the
__weak.  region_is_ram() needs neither so yes, they should be removed.

<looks at the code>

Doing strcmp("System RAM") is rather a hack.  Is there nothing in
resource.flags which can be used?  Or added otherwise?