x86/agp: Fix agp_amd64_init() initialization with CONFIG_GART_IOMMU enabled

with CONFIG_GART_IOMMU enabled drivers/char/agp/amd64-agp.c has:

 #ifndef CONFIG_GART_IOMMU
 module_init(agp_amd64_init);
 module_exit(agp_amd64_cleanup);
 #endif

agp_amd64_init() was called via gart_iommu_init with
CONFIG_GART_IOMMU=y agp_amd64_init() was called via module_init
with CONFIG_GART_IOMMU=n

The commit 75f1cdf1 changes the
x86 dma initialization routine: gart_iommu_init() is called only
when GART IOMMU is detected. So when GART IOMMU isn't detected,
agp_amd64_init isn't called.

Marin Mitov reported this issue:

 http://marc.info/?l=linux-kernel&m=126192729110083&w=2

With this patch, agp_amd64_init() is always called via
module_init (the above ifndef is removed). If agp_amd64_init()
is called via gart_iommu_init() earlier, agp_amd64_init()
finishes without doing anything (when it is called via
module_init).
Reported-by: NMarin Mitov <mitov@issp.bas.bg>
Tested-by: NMarin Mitov <mitov@issp.bas.bg>
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: davej@redhat.com
LKML-Reference: <20091228181118C.fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

drivers/char/agp/amd64-agp.c

@@ -725,9 +725,14 @@ static struct pci_driver agp_amd64_pci_driver = {
 int __init agp_amd64_init(void)
 {
 	int err = 0;
+	static int done = 0;
 
 	if (agp_off)
 		return -EINVAL;
+
+	if (done++)
+		return agp_bridges_found ? 0 : -ENODEV;
+
 	err = pci_register_driver(&agp_amd64_pci_driver);
 	if (err < 0)
 		return err;
@@ -771,12 +776,8 @@ static void __exit agp_amd64_cleanup(void)
 	pci_unregister_driver(&agp_amd64_pci_driver);
 }
 
-/* On AMD64 the PCI driver needs to initialize this driver early
-   for the IOMMU, so it has to be called via a backdoor. */
-#ifndef CONFIG_GART_IOMMU
 module_init(agp_amd64_init);
 module_exit(agp_amd64_cleanup);
-#endif
 
 MODULE_AUTHOR("Dave Jones <davej@redhat.com>, Andi Kleen");
 module_param(agp_try_unsupported, bool, 0);