[PATCH] [2/2] POWERPC: Lower threshold for DART enablement to 1GB

[Olof Johansson]

Hi,

This goes with the previous dma_mask patch -- enable DART at 1GB due to
Airport Extreme cards needing it. Please consider for 2.6.17.


Thanks,

Olof


---

Turn on the DART already at 1GB. This is needed because of crippled
devices in some systems, i.e. Airport Extreme cards, only supporting
30-bit DMA addresses.

Otherwise, users with between 1 and 2GB of memory will need to manually
enable it with iommu=force, and that's no good.

Some simple performance tests show that there's a slight impact of
enabling DART, but it's in the 1-3% range (kernel build with disk I/O
as well as over NFS).

iommu=off can still be used for those who don't want to deal with the
overhead (and don't need it for any devices).


Signed-off-by: Olof Johansson <olof at lixom.net>

Index: 2.6/arch/powerpc/sysdev/dart_iommu.c
===================================================================
--- 2.6.orig/arch/powerpc/sysdev/dart_iommu.c
+++ 2.6/arch/powerpc/sysdev/dart_iommu.c
@@ -329,10 +329,14 @@ void iommu_init_early_dart(void)
 
 void __init alloc_dart_table(void)
 {
-	/* Only reserve DART space if machine has more than 2GB of RAM
+	/* Only reserve DART space if machine has more than 1GB of RAM
 	 * or if requested with iommu=on on cmdline.
+	 *
+	 * 1GB of RAM is picked as limit because some default devices
+	 * (i.e. Airport Extreme) have 30 bit address range limits.
 	 */
-	if (lmb_end_of_DRAM() <= 0x80000000ull && !iommu_force_on)
+
+	if (lmb_end_of_DRAM() <= 0x40000000ull && !iommu_force_on)
 		return;
 
 	/* 512 pages (2MB) is max DART tablesize. */