x86, mm: Allow ZONE_DMA to be configurable

ZONE_DMA is unnecessary for a large number of machines that do not
require less than 32-bit DMA addressing, e.g. ISA legacy DMA or PCI
cards with a restricted DMA address mask.

This patch allows users to disable ZONE_DMA for x86 if they know they
will not be using such devices with their kernel.

This prevents the VM from unnecessarily reserving a ratio of memory
(defaulting to 1/256th of system capacity) with lowmem_reserve_ratio
for such allocations when it will never be used.

Signed-off-by: David Rientjes <rientjes@google.com>
Link: http://lkml.kernel.org/r/alpine.DEB.2.00.1105161353560.4353@chino.kir.corp.google.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

3 files changed, 12 insertions, 1 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 648fca42ae6a..0eb801a75dee 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -112,7 +112,14 @@ config MMU
         def_bool y
 
 config ZONE_DMA
-        def_bool y
+        bool "DMA memory allocation support" if EXPERT
+        default y
+        help
+          DMA memory allocation support allows devices with less than 32-bit
+          addressing to allocate within the first 16MB of address space.
+          Disable if no such devices will be used.
+
+          If unsure, say Y.
 
 config SBUS
         bool
diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
index 2cde0a34bed6..29f7c6d98179 100644
--- a/arch/x86/mm/init_32.c
+++ b/arch/x86/mm/init_32.c
@@ -678,8 +678,10 @@ static void __init zone_sizes_init(void)
 {
         unsigned long max_zone_pfns[MAX_NR_ZONES];
         memset(max_zone_pfns, 0, sizeof(max_zone_pfns));
+#ifdef CONFIG_ZONE_DMA
         max_zone_pfns[ZONE_DMA] =
                 virt_to_phys((char *)MAX_DMA_ADDRESS) >> PAGE_SHIFT;
+#endif
         max_zone_pfns[ZONE_NORMAL] = max_low_pfn;
 #ifdef CONFIG_HIGHMEM
         max_zone_pfns[ZONE_HIGHMEM] = highend_pfn;
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 0404bb3a077e..d865c4aeec55 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -616,7 +616,9 @@ void __init paging_init(void)
         unsigned long max_zone_pfns[MAX_NR_ZONES];
 
         memset(max_zone_pfns, 0, sizeof(max_zone_pfns));
+#ifdef CONFIG_ZONE_DMA
         max_zone_pfns[ZONE_DMA] = MAX_DMA_PFN;
+#endif
         max_zone_pfns[ZONE_DMA32] = MAX_DMA32_PFN;
         max_zone_pfns[ZONE_NORMAL] = max_pfn;