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Date: Mon, 24 Sep 2018 13:41:48 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: block: DMA alignment of IO buffer allocated from slab
Message-ID: <20180924204148.GA2542@bombadil.infradead.org>
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To: Bart Van Assche <bvanassche@acm.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>, Ming Lei <ming.lei@redhat.com>, Vitaly Kuznetsov <vkuznets@redhat.com>, Christoph Hellwig <hch@lst.de>, Ming Lei <tom.leiming@gmail.com>, linux-block <linux-block@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, Linux FS Devel <linux-fsdevel@vger.kernel.org>, "open list:XFS FILESYSTEM" <linux-xfs@vger.kernel.org>, Dave Chinner <dchinner@redhat.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Jens Axboe <axboe@kernel.dk>, Christoph Lameter <cl@linux.com>, Linus Torvalds <torvalds@linux-foundation.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>

On Mon, Sep 24, 2018 at 12:56:18PM -0700, Bart Van Assche wrote:
> On Mon, 2018-09-24 at 11:57 -0700, Matthew Wilcox wrote:
> > You're not supposed to use kmalloc memory for DMA.  This is why we have
> > dma_alloc_coherent() and friends.
> 
> Are you claiming that all drivers that use DMA should use coherent DMA only? If
> coherent DMA is the only DMA style that should be used, why do the following
> function pointers exist in struct dma_map_ops?

Good job snipping the part of my reply which addressed this.  Go read
DMA-API.txt yourself.  Carefully.