linux-kernel - Re: [PATCH 07/11] powerpc: make dma_cache

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Message-ID: <fd0ccac1-51dc-1eff-f1ac-63f9df282de2@c-s.fr>
Date:   Wed, 4 Oct 2017 10:29:13 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org, Chris Zankel <chris@...kel.net>,
        Michal Simek <monstr@...str.eu>, linux-ia64@...r.kernel.org,
        linux-mips@...ux-mips.org, linux-sh@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-xtensa@...ux-xtensa.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        David Howells <dhowells@...hat.com>,
        Max Filippov <jcmvbkbc@...il.com>,
        Guan Xuetao <gxt@...c.pku.edu.cn>,
        Robin Murphy <robin.murphy@....com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH 07/11] powerpc: make dma_cache_sync a no-op



Le 03/10/2017 à 13:43, Christoph Hellwig a écrit :
> On Tue, Oct 03, 2017 at 01:24:57PM +0200, Christophe LEROY wrote:
>>> powerpc does not implement DMA_ATTR_NON_CONSISTENT allocations, so it
>>> doesn't make any sense to do any work in dma_cache_sync given that it
>>> must be a no-op when dma_alloc_attrs returns coherent memory.
>> What about arch/powerpc/mm/dma-noncoherent.c ?
>>
>> Powerpc 8xx doesn't have coherent memory.
> 
> It doesn't implement the DMA_ATTR_NON_CONSISTENT interface either,
> so if it really doesn't have a way to provide dma coherent allocation
> (although the code in __dma_alloc_coherent suggests it does provide
> dma coherent allocations) I have no idea how it could ever have
> worked.
> 

Yes indeed it provides coherent memory by allocation non cached memory.

And drivers aiming at using non coherent memory do it by using kmalloc() 
with GFP_DMA then dma_map_single().
Then they use dma_sync_single_for_xxx(), which calls __dma_sync() on the 
8xx and is a nop on other powerpcs.

Christophe