[PATCH] powerpc: Lower threshold for DART enablement to 1GB

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@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>

diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index 4336390bcf34e86671b12e5e4b66594b9f401e68..1cb69e8fb0b10139f2f83531854f40cde57b63a0 100644 (file)
--- a/arch/powerpc/kernel/prom.c
+++ b/arch/powerpc/kernel/prom.c
@@ -62,7 +62,7 @@ static int __initdata dt_root_addr_cells;
 static int __initdata dt_root_size_cells;
 
 #ifdef CONFIG_PPC64
-static int __initdata iommu_is_off;
+int __initdata iommu_is_off;
 int __initdata iommu_force_on;
 unsigned long tce_alloc_start, tce_alloc_end;
 #endif

diff --git a/arch/powerpc/sysdev/dart_iommu.c b/arch/powerpc/sysdev/dart_iommu.c
index 61d317428610ab3d953211366d2e938284f0d72a..38087bd6e3cf4273d5efeb38baa68ccf90328245 100644 (file)
--- a/arch/powerpc/sysdev/dart_iommu.c
+++ b/arch/powerpc/sysdev/dart_iommu.c
@@ -49,6 +49,7 @@
 
 #include "dart.h"
 
+extern int iommu_is_off;
 extern int iommu_force_on;
 
 /* Physical base address and size of the DART table */
@@ -329,10 +330,17 @@ 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 (iommu_is_off)
+               return;
+
+       if (!iommu_force_on && lmb_end_of_DRAM() <= 0x40000000ull)
                return;
 
        /* 512 pages (2MB) is max DART tablesize. */