linux-kernel - Re: [PATCH] dma_declare_coherent

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Message-ID: <Pine.LNX.4.60.0704232239250.12939@poirot.grange>
Date:	Mon, 23 Apr 2007 22:45:21 +0200 (CEST)
From:	Guennadi Liakhovetski <g.liakhovetski@....de>
To:	James Bottomley <James.Bottomley@...elEye.com>
cc:	linux-kernel@...r.kernel.org, jayalk@...works.biz,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] dma_declare_coherent_memory wrong allocation

On Mon, 23 Apr 2007, James Bottomley wrote:

> On Mon, 2007-04-23 at 00:45 +0200, Guennadi Liakhovetski wrote:
> > Right, thinko. How about using his:
> > 
> > +	int pages = DIV_ROUND_UP(size, PAGE_SIZE);
> 
> Actually, no ... this has to be size >> PAGE_SHIFT.  The reason being
> that the  allocator is designed to allocate pages out of a device memory
> buffer.  If the size isn't a multiple of page size, we have to round
> down (we can't allocate the last page if the memory we have is only part
> of a page).
> 
> I suppose if you want to catch the unlikely nutcase where size <
> PAGE_SIZE you could
> 
> if (unlikely(pages == 0))
> 	goto out;
> 
> > +	int bitmap_size = BITS_TO_LONGS(pages) * BYTES_PER_LONG;
> 
> This is fine, except the BYTES_PER_LONG.  Traditionally, we do this with
> sizeof(long).  The only reason to have a #define for it is if it has to
> be used in a macro (the compiler does sizeof() not the preprocessor).
> 
> How about just a simple
> 
> int bitmap_size = BITS_TO_LONGS(pages) * sizeof(long);

Ok, version 3, small, and hopefully, beautiful:

dma_declare_coherent_memory() allocates a bitmap 1 bit per page, it 
calculates the bitmap size based on size of long, but allocates bytes...
Thanks to James Bottomley for clarifications and corrections.

Signed-off-by: G. Liakhovetski <g.liakhovetski@....de>

diff --git a/arch/cris/arch-v32/drivers/pci/dma.c b/arch/cris/arch-v32/drivers/pci/dma.c
index 70d3bf0..832fc63 100644
--- a/arch/cris/arch-v32/drivers/pci/dma.c
+++ b/arch/cris/arch-v32/drivers/pci/dma.c
@@ -76,7 +76,7 @@ int dma_declare_coherent_memory(struct device *dev, dma_addr_t bus_addr,
 {
 	void __iomem *mem_base;
 	int pages = size >> PAGE_SHIFT;
-	int bitmap_size = (pages + 31)/32;
+	int bitmap_size = BITS_TO_LONGS(pages) * sizeof(long);
 
 	if ((flags & (DMA_MEMORY_MAP | DMA_MEMORY_IO)) == 0)
 		goto out;
diff --git a/arch/i386/kernel/pci-dma.c b/arch/i386/kernel/pci-dma.c
index 3ebcea0..30b754f 100644
--- a/arch/i386/kernel/pci-dma.c
+++ b/arch/i386/kernel/pci-dma.c
@@ -77,7 +77,7 @@ int dma_declare_coherent_memory(struct device *dev, dma_addr_t bus_addr,
 {
 	void __iomem *mem_base = NULL;
 	int pages = size >> PAGE_SHIFT;
-	int bitmap_size = (pages + 31)/32;
+	int bitmap_size = BITS_TO_LONGS(pages) * sizeof(long);
 
 	if ((flags & (DMA_MEMORY_MAP | DMA_MEMORY_IO)) == 0)
 		goto out;

-
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