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

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Date:   Tue, 3 Oct 2017 12:43:07 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christophe LEROY <christophe.leroy@....fr>,
        Christoph Hellwig <hch@....de>,
        iommu@...ts.linux-foundation.org
Cc:     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>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH 07/11] powerpc: make dma_cache_sync a no-op

On 03/10/17 12:24, Christophe LEROY wrote:
> 
> 
> Le 03/10/2017 à 12:43, Christoph Hellwig a écrit :
>> 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 ?

AFAICS, just like the ARM code from which it is derived, that will
always return a non-cacheable remap of the allocation, which should thus
not require explicit maintenance after CPU accesses. dma_cache_sync()
would only matter if dma_alloc_attrs(..., DMA_ATTR_NON_CONSISTENT) got
special treatment and was allowed to return a cacheable address, but PPC
doesn't even propagate the attrs to its internal __dma_alloc_coherent()
implementations.

Robin.

> Powerpc 8xx doesn't have coherent memory.
> 
> Christophe
> 
>>
>> Signed-off-by: Christoph Hellwig <hch@....de>
>> ---
>>   arch/powerpc/include/asm/dma-mapping.h | 2 --
>>   1 file changed, 2 deletions(-)
>>
>> diff --git a/arch/powerpc/include/asm/dma-mapping.h
>> b/arch/powerpc/include/asm/dma-mapping.h
>> index eaece3d3e225..320846442bfb 100644
>> --- a/arch/powerpc/include/asm/dma-mapping.h
>> +++ b/arch/powerpc/include/asm/dma-mapping.h
>> @@ -144,8 +144,6 @@ static inline phys_addr_t dma_to_phys(struct
>> device *dev, dma_addr_t daddr)
>>   static inline void dma_cache_sync(struct device *dev, void *vaddr,
>> size_t size,
>>           enum dma_data_direction direction)
>>   {
>> -    BUG_ON(direction == DMA_NONE);
>> -    __dma_sync(vaddr, size, (int)direction);
>>   }
>>     #endif /* __KERNEL__ */
>>