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Date:   Sat, 19 Feb 2022 11:18:24 +0000
From:   Hyeonggon Yoo <42.hyeyoo@gmail.com>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Baoquan He <bhe@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, akpm@linux-foundation.org, cl@linux.com,
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        vbabka@suse.cz, David.Laight@aculab.com, david@redhat.com,
        herbert@gondor.apana.org.au, davem@davemloft.net,
        linux-crypto@vger.kernel.org, steffen.klassert@secunet.com,
        netdev@vger.kernel.org, hca@linux.ibm.com, gor@linux.ibm.com,
        agordeev@linux.ibm.com, borntraeger@linux.ibm.com,
        svens@linux.ibm.com, linux-s390@vger.kernel.org, michael@walle.cc,
        linux-i2c@vger.kernel.org, wsa@kernel.org
Subject: Re: [PATCH 22/22] mtd: rawnand: Use dma_alloc_noncoherent() for dma
 buffer
Message-ID: <YhDSAJG+LksZSnLP@ip-172-31-19-208.ap-northeast-1.compute.internal>
References: <20220219005221.634-1-bhe@redhat.com>
 <20220219005221.634-23-bhe@redhat.com>
 <20220219071900.GH26711@lst.de>
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On Sat, Feb 19, 2022 at 08:19:00AM +0100, Christoph Hellwig wrote:
> On Sat, Feb 19, 2022 at 08:52:21AM +0800, Baoquan He wrote:
> > Use dma_alloc_noncoherent() instead of directly allocating buffer
> > from kmalloc with GFP_DMA. DMA API will try to allocate buffer
> > depending on devices addressing limitation.
> 
> 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.

At this point I thought we need an API that allocates based on
address bit mask (like dma_alloc_noncoherent()), which does not maps buffer
into dma address. __get_free_pages/kmalloc(GFP_DMA) has been so confusing..

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

Thanks,
Hyeonggon