From: James Bottomley <James.Bottomley@SteelEye.com>

This patch implements dma_get_required_mask() which may be used by drivers
to probe the optimal DMA descriptor type they should be implementing on the
platform.

I've also tested it this time with the sym_2 driver...making it chose the
correct descriptors for the platform.  (although I don't have a 64 bit
platform with >4GB memory, so I only confirmed it selects the 32 bit
descriptors all the time...)

Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 25-akpm/Documentation/DMA-API.txt   |   14 ++++++++++++++
 25-akpm/include/linux/dma-mapping.h |   23 +++++++++++++++++++++++
 25-akpm/mm/bootmem.c                |    5 +++++
 3 files changed, 42 insertions(+)

diff -puN Documentation/DMA-API.txt~dma_get_required_mask Documentation/DMA-API.txt
--- 25/Documentation/DMA-API.txt~dma_get_required_mask	2004-06-28 17:37:35.129480896 -0700
+++ 25-akpm/Documentation/DMA-API.txt	2004-06-28 17:37:35.137479680 -0700
@@ -162,6 +162,20 @@ parameters if it is.
 
 Returns: 1 if successful and 0 if not
 
+u64
+dma_get_required_mask(struct device *dev)
+
+After setting the mask with dma_set_mask(), this API returns the
+actual mask (within that already set) that the platform actually
+requires to operate efficiently.  Usually this means the returned mask
+is the minimum required to cover all of memory.  Examining the
+required mask gives drivers with variable descriptor sizes the
+opportunity to use smaller descriptors as necessary.
+
+Requesting the required mask does not alter the current mask.  If you
+wish to take advantage of it, you should issue another dma_set_mask()
+call to lower the mask again.
+
 
 Part Id - Streaming DMA mappings
 --------------------------------
diff -puN include/linux/dma-mapping.h~dma_get_required_mask include/linux/dma-mapping.h
--- 25/include/linux/dma-mapping.h~dma_get_required_mask	2004-06-28 17:37:35.131480592 -0700
+++ 25-akpm/include/linux/dma-mapping.h	2004-06-28 17:37:35.137479680 -0700
@@ -19,6 +19,29 @@ enum dma_data_direction {
 #define dma_sync_single		dma_sync_single_for_cpu
 #define dma_sync_sg		dma_sync_sg_for_cpu
 
+#ifndef ARCH_HAS_DMA_GET_REQUIRED_MASK
+static inline u64 dma_get_required_mask(struct device *dev)
+{
+	extern unsigned long max_pfn; /* defined in bootmem.h but may
+					 not be included */
+	u32 low_totalram = ((max_pfn - 1) << PAGE_SHIFT);
+	u32 high_totalram = ((max_pfn - 1) >> (32 - PAGE_SHIFT));
+	u64 mask;
+
+	if (!high_totalram) {
+		/* convert to mask just covering totalram */
+		low_totalram = (1 << (fls(low_totalram) - 1));
+		low_totalram += low_totalram - 1;
+		mask = low_totalram;
+	} else {
+		high_totalram = (1 << (fls(high_totalram) - 1));
+		high_totalram += high_totalram - 1;
+		mask = (((u64)high_totalram) << 32) + 0xffffffff;
+	}
+	return mask & *dev->dma_mask;
+}
+#endif
+
 #endif
 
 
diff -puN mm/bootmem.c~dma_get_required_mask mm/bootmem.c
--- 25/mm/bootmem.c~dma_get_required_mask	2004-06-28 17:37:35.133480288 -0700
+++ 25-akpm/mm/bootmem.c	2004-06-28 17:37:35.138479528 -0700
@@ -16,6 +16,7 @@
 #include <linux/init.h>
 #include <linux/bootmem.h>
 #include <linux/mmzone.h>
+#include <linux/module.h>
 #include <asm/dma.h>
 #include <asm/io.h>
 
@@ -27,6 +28,10 @@ unsigned long max_low_pfn;
 unsigned long min_low_pfn;
 unsigned long max_pfn;
 
+EXPORT_SYMBOL(max_pfn);		/* This is exported so
+				 * dma_get_required_mask(), which uses
+				 * it, can be an inline function */
+
 /* return the number of _pages_ that will be allocated for the boot bitmap */
 unsigned long __init bootmem_bootmap_pages (unsigned long pages)
 {
_