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To: linux-kernel@vger.kernel.org
From: Timur Tabi <timur@codeaurora.org>
Subject: Proper way to allocate DMA buffer within a single 4GB block?
Message-ID: <cc400aeb-e2ef-748d-b579-917eeb1c44f8@codeaurora.org>
Date: Wed, 20 Sep 2017 17:18:09 -0500
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I have a device that requires I allocated a few buffers for DMA. The 
problem is that this device has only one register for the upper 32 bits 
of all of the buffers. That is, all of buffers must reside within the 
same 4GB block of memory. In order words,

	end = start + size - 1;
	if (upper_32_bits(start) != upper_32_bits(end))
		// Oh no, the buffer spans across a 4GB boundary!

The buffer is typically less than 16KB in size, so we've never seen it 
actually span across a 4GB boundary.  However, I want to ensure that 
it's impossible.  I wrote this function that re-tries the allocation if 
the first one is invalid, but I suspect that it's too hackish.  Is there 
a better way?

static void *dma_alloc_4gb_block(struct device *dev, size_t size,
				   dma_addr_t *dma_handle)
{
	dma_addr_t start, start2, end;
	void *v, *v2;

	v = dma_zalloc_coherent(dev, size, &start, GFP_KERNEL);
	if (!v)
		return NULL;

	/* Test to see if the entire buffer is within one 4G block */
	end = start + size - 1;
	if (upper_32_bits(start) == upper_32_bits(end)) {
		*dma_handle = start;
		return v;
	}

	/* The DMA buffer spanned a 32-bit boundry!  Allocate another
	 * block and try again.  Assuming this block as adjacent to
	 * the first one, it should be valid.
	 */
	v2 = dma_zalloc_coherent(dev, size, &start2, GFP_KERNEL);
	dma_free_coherent(dev, size, v, start);
	if (!v2) {
		return NULL;
	}

	/* Test to see if the entire buffer is within one 4G block */
	end = start2 + size - 1;
	if (upper_32_bits(start2) == upper_32_bits(end)) {
		*dma_handle = start2;
		return v2;
	}

	/* We tried twice and still failed, so just give up. */
	dma_free_coherent(dev, size, v2, start2);
	return NULL;
}

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