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Subject: Re: block: DMA alignment of IO buffer allocated from slab
From: Bart Van Assche <bvanassche@acm.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>, Ming Lei
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Date: Mon, 24 Sep 2018 13:54:01 -0700
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On Mon, 2018-09-24 at 13:41 -0700, Matthew Wilcox wrote:
+AD4 On Mon, Sep 24, 2018 at 12:56:18PM -0700, Bart Van Assche wrote:
+AD4 +AD4 On Mon, 2018-09-24 at 11:57 -0700, Matthew Wilcox wrote:
+AD4 +AD4 +AD4 You're not supposed to use kmalloc memory for DMA.  This is why we have
+AD4 +AD4 +AD4 dma+AF8-alloc+AF8-coherent() and friends.
+AD4 +AD4 
+AD4 +AD4 Are you claiming that all drivers that use DMA should use coherent DMA only? If
+AD4 +AD4 coherent DMA is the only DMA style that should be used, why do the following
+AD4 +AD4 function pointers exist in struct dma+AF8-map+AF8-ops?
+AD4 
+AD4 Good job snipping the part of my reply which addressed this.  Go read
+AD4 DMA-API.txt yourself.  Carefully.

The snipped part did not contradict your claim that +ACI-You're not supposed to use
kmalloc memory for DMA.+ACI In the DMA-API.txt document however there are multiple
explicit statements that support allocating memory for DMA with kmalloc(). Here
is one example from the DMA-API.txt section about dma+AF8-map+AF8-single():

	Not all memory regions in a machine can be mapped by this API.
	Further, contiguous kernel virtual space may not be contiguous as
	physical memory.  Since this API does not provide any scatter/gather
	capability, it will fail if the user tries to map a non-physically
	contiguous piece of memory.  For this reason, memory to be mapped by
	this API should be obtained from sources which guarantee it to be
	physically contiguous (like kmalloc).

Bart.