netdev - RE: [PATCH 22/22] mtd: rawnand: Use dma_alloc

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Date:   Tue, 22 Feb 2022 09:06:48 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christoph Hellwig' <hch@....de>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>
CC:     Baoquan He <bhe@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "cl@...ux.com" <cl@...ux.com>,
        "penberg@...nel.org" <penberg@...nel.org>,
        "rientjes@...gle.com" <rientjes@...gle.com>,
        "iamjoonsoo.kim@....com" <iamjoonsoo.kim@....com>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "david@...hat.com" <david@...hat.com>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
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        "wsa@...nel.org" <wsa@...nel.org>
Subject: RE: [PATCH 22/22] mtd: rawnand: Use dma_alloc_noncoherent() for dma
 buffer

From: Christoph Hellwig
> Sent: 22 February 2022 08:47
...
> > 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.

For systems with an iommu that should save all the iommu setup
for every transfer.
I'd also guess that it saves worrying about the error path
when the dma_map fails (eg because the iommu has no space).

OTOH the driver would be 'hogging' iommu space, so maybe
allocate during open() (or equivalent).

	David

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