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Date:   Tue, 22 Feb 2022 09:46:52 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc:     Christoph Hellwig <hch@lst.de>, Baoquan He <bhe@redhat.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
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Subject: Re: [PATCH 22/22] mtd: rawnand: Use dma_alloc_noncoherent() for
 dma buffer
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References: <20220219005221.634-1-bhe@redhat.com> <20220219005221.634-23-bhe@redhat.com> <20220219071900.GH26711@lst.de> <YhDSAJG+LksZSnLP@ip-172-31-19-208.ap-northeast-1.compute.internal>
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On Sat, Feb 19, 2022 at 11:18:24AM +0000, Hyeonggon Yoo wrote:
> > I think it would be better to still allocate the buffer at allocation
> > time and then just transfer ownership using dma_sync_single* in the I/O
> > path to avoid the GFP_ATOMIC allocation.
> 
> This driver allocates the buffer at initialization step and maps the buffer
> for DMA_TO_DEVICE and DMA_FROM_DEVICE when processing IO.
> 
> But after making this driver to use dma_alloc_noncoherent(), remapping
> dma_alloc_noncoherent()-ed buffer is strange So I just made it to allocate
> the buffer in IO path.

You should not remap it.  Just use dma_sync_single* to transfer ownership.

> Hmm.. for this specific case, What about allocating two buffers
> for DMA_TO_DEVICE and DMA_FROM_DEVICE at initialization time?

That will work, but I don't see the benefit as you'd still need to call
dma_sync_single* before and after each data transfer.