[PATCH 1/3] Add dma_set_mask_weak() API

[Michael Buesch]

This adds a new DMA subsystem API call for "weak" setting
of the DMA mask.
Weak means that it will fallback to smaller masks in case the
DMA subsystem rejects a big mask.
Currently such rejection may happen if the driver requests a 64bit
mask on a VIA machine, for example. dma_set_mask_weak() will fallback
to 32bit, in that case, and tell the caller about it by modifying the
passed mask.

Signed-off-by: Michael Buesch <mb@xxxxxxxxx>


Index: wireless-testing/drivers/base/dma-mapping.c
===================================================================
--- wireless-testing.orig/drivers/base/dma-mapping.c	2008-04-28 23:34:19.000000000 +0200
+++ wireless-testing/drivers/base/dma-mapping.c	2008-04-28 23:46:25.000000000 +0200
@@ -216,3 +216,38 @@ void dmam_release_declared_memory(struct
 EXPORT_SYMBOL(dmam_release_declared_memory);
 
 #endif
+
+/**
+ * dma_set_mask_weak - Set the DMA mask. Retry with smaller masks.
+ * @dev: Device to set the mask on.
+ * @mask: Pointer to the mask that you want to set. The function will
+ * 	modify the mask and set it to the actually used mask, in case
+ * 	it had to fall back to a smaller mask.
+ *
+ * Set the DMA mask and allow falling back to smaller masks in
+ * case of an error.
+ *
+ * RETURNS:
+ * 0 on success, -errno on failure.
+ */
+int dma_set_mask_weak(struct device *dev, u64 *mask)
+{
+	u64 m = *mask;
+	int err;
+
+	if (m < DMA_MIN_FALLBACK_MASK)
+		return -EINVAL;
+	while (1) {
+		err = dma_set_mask(dev, m);
+		if (!err)
+			break;
+		/* Did not like this one. Try a smaller one. */
+		m >>= 1;
+		if (m < DMA_MIN_FALLBACK_MASK)
+			return err;
+	}
+	*mask = m;
+
+	return 0;
+}
+EXPORT_SYMBOL(dma_set_mask_weak);
Index: wireless-testing/include/linux/dma-mapping.h
===================================================================
--- wireless-testing.orig/include/linux/dma-mapping.h	2008-04-28 23:34:19.000000000 +0200
+++ wireless-testing/include/linux/dma-mapping.h	2008-04-28 23:35:35.000000000 +0200
@@ -60,6 +60,11 @@ static inline int is_device_dma_capable(
 
 extern u64 dma_get_required_mask(struct device *dev);
 
+extern int dma_set_mask_weak(struct device *dev, u64 *mask);
+/* Smallest mask fallback used by dma_set_mask_weak(). */
+#define DMA_MIN_FALLBACK_MASK	DMA_BIT_MASK(24) /* 16 MB */
+
+
 static inline unsigned int dma_get_max_seg_size(struct device *dev)
 {
 	return dev->dma_parms ? dev->dma_parms->max_segment_size : 65536;


-- 
Greetings Michael.
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