x86/PCI: allocate space from the end of a region, not the beginning

Allocate from the end of a region, not the beginning.

For example, if we need to allocate 0x800 bytes for a device on bus
0000:00 given these resources:

    [mem 0xbff00000-0xdfffffff] PCI Bus 0000:00
      [mem 0xc0000000-0xdfffffff] PCI Bus 0000:02

the available space at [mem 0xbff00000-0xbfffffff] is passed to the
alignment callback (pcibios_align_resource()).  Prior to this patch, we
would put the new 0x800 byte resource at the beginning of that available
space, i.e., at [mem 0xbff00000-0xbff007ff].

With this patch, we put it at the end, at [mem 0xbffff800-0xbfffffff].

Reference: https://bugzilla.kernel.org/show_bug.cgi?id=16228#c41Signed-off-by: NBjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>

arch/x86/pci/i386.c

@@ -65,16 +65,21 @@ pcibios_align_resource(void *data, const struct resource *res,
 			resource_size_t size, resource_size_t align)
 {
 	struct pci_dev *dev = data;
-	resource_size_t start = res->start;
+	resource_size_t start = round_down(res->end - size + 1, align);
 
 	if (res->flags & IORESOURCE_IO) {
-		if (skip_isa_ioresource_align(dev))
-			return start;
-		if (start & 0x300)
-			start = (start + 0x3ff) & ~0x3ff;
+
+		/*
+		 * If we're avoiding ISA aliases, the largest contiguous I/O
+		 * port space is 256 bytes.  Clearing bits 9 and 10 preserves
+		 * all 256-byte and smaller alignments, so the result will
+		 * still be correctly aligned.
+		 */
+		if (!skip_isa_ioresource_align(dev))
+			start &= ~0x300;
 	} else if (res->flags & IORESOURCE_MEM) {
 		if (start < BIOS_END)
-			start = BIOS_END;
+			start = res->end;	/* fail; no space */
 	}
 	return start;
 }