linux-kernel - Re: [PATCH 22/22] mtd: rawnand: Use dma_alloc

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Date:   Sat, 19 Feb 2022 11:18:24 +0000
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Baoquan He <bhe@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org, cl@...ux.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        vbabka@...e.cz, David.Laight@...lab.com, david@...hat.com,
        herbert@...dor.apana.org.au, davem@...emloft.net,
        linux-crypto@...r.kernel.org, steffen.klassert@...unet.com,
        netdev@...r.kernel.org, hca@...ux.ibm.com, gor@...ux.ibm.com,
        agordeev@...ux.ibm.com, borntraeger@...ux.ibm.com,
        svens@...ux.ibm.com, linux-s390@...r.kernel.org, michael@...le.cc,
        linux-i2c@...r.kernel.org, wsa@...nel.org
Subject: Re: [PATCH 22/22] mtd: rawnand: Use dma_alloc_noncoherent() for dma
 buffer

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