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Re: Correct way to set coherent_dma_mask on a non-pci device?

Subject: Re: Correct way to set coherent_dma_mask on a non-pci device?
From: Steve Brown <>
Date: Thu, 10 Jul 2008 13:38:25 -0400
Cc: Michael Buesch <>
In-reply-to: <>
Original-recipient: rfc822;
References: <> <>
User-agent: Thunderbird (X11/20080501)
Michael Buesch wrote:
On Tuesday 08 July 2008 19:40:06 Steve Brown wrote:
There appears to be no function like pci_set_consistent_dma_mask to set the coherent mask for a non-pci device.

What is the "proper" way to set it?

The context for the question is a recent change to ssb_dma_set_mask() in drivers/ssb/main.c that removed the somewhat fragile, direct manipulation of dma_mask and coherent_dma_mask in favor of a call to dma_set_mask().

Note that SSB devices use the dma_*** API for doing DMA remappings.
So it uses dma_set_mask() for setting the mask.

I can't find any dma_*** routine that references coherent_dma_mask. It looks like dma_set_mask() doesn't handle the case where the device doesn't support coherent dma (CONFIG_DMA_NONCOHERENT=y).

Would this be the correct patch to handle that case?


diff --git a/include/asm-mips/dma-mapping.h b/include/asm-mips/dma-mapping.h
index 230b3f1..8da4107 100644
--- a/include/asm-mips/dma-mapping.h
+++ b/include/asm-mips/dma-mapping.h
@@ -1,8 +1,9 @@
-#include <asm/scatterlist.h>
+#include <linux/scatterlist.h>
 #include <asm/cache.h>
+#include <dma-coherence.h>
 void *dma_alloc_noncoherent(struct device *dev, size_t size,
                           dma_addr_t *dma_handle, gfp_t flag);
@@ -48,7 +49,11 @@ extern int dma_supported(struct device *dev, u64 mask);
 static inline int
 dma_set_mask(struct device *dev, u64 mask)
-       if(!dev->dma_mask || !dma_supported(dev, mask))
+       if(!dma_supported(dev, mask))
+               return -EIO;
+       if(!plat_device_is_coherent(dev))
+               dev->dma_mask = &dev->coherent_dma_mask;
+       if(!dev->dma_mask)
                return -EIO;
        *dev->dma_mask = mask;
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