drivers: Add option to reserve default CMA region

CMA provides good utilization of memory but for some use cases, the
allocation time is too costly. Add a Kconfig option to allow the
default region to be permanently set aside for contiguous use cases.

Change-Id: I1eef508d37cf6ae3b7b7a652fc59391b186fc122
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>

drivers/base/Kconfig

@@ -290,6 +290,19 @@ config CMA_AREAS
 
 	  If unsure, leave the default value "7".
 
+config CMA_RESERVE_DEFAULT_AREA
+	bool "Permanently reserve the default area"
+	help
+	  By default, contiguous regions are given back to the page allocator so
+	  the memory may be used when not in contiguous forms. This has better
+	  utilization but the allocation time may be slower. Say Y here to
+	  never have the default region be returned to the page allocator.
+	  This means the memory will always be available for contiguous
+	  allocations but the pages will never be available for anything else
+	  except contiguous allocations.
+
+	  If unsure, say n.
+
 endif
 
 endmenu

drivers/base/dma-contiguous.c

@@ -97,6 +97,11 @@ static struct cma *cma_get_area_by_name(const char *name)
 #define CMA_SIZE_MBYTES 0
 #endif
 
+#ifdef CONFIG_CMA_RESERVE_DEFAULT_AREA
+#define CMA_RESERVE_AREA 1
+#else
+#define CMA_RESERVE_AREA 0
+#endif
 /*
  * Default global CMA area size can be defined in kernel's .config.
  * This is usefull mainly for distro maintainers to create a kernel
@@ -275,7 +280,7 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
 			 (unsigned long)sel_size / SZ_1M);
 
 		if (dma_contiguous_reserve_area(sel_size, &base, limit, NULL,
-		    true) == 0)
+		    CMA_RESERVE_AREA) == 0)
 			dma_contiguous_def_base = base;
 	}
 #ifdef CONFIG_OF