linux-kernel - Re: [PATCH] arm64: Increase the max granular size

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Message-ID: <20151105121706.GQ7637@e104818-lin.cambridge.arm.com>
Date:	Thu, 5 Nov 2015 12:17:06 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Joonsoo Kim <js1304@...il.com>
Cc:	Robert Richter <rric@...nel.org>,
	Will Deacon <will.deacon@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Robert Richter <rrichter@...ium.com>,
	Tirumalesh Chalamarla <tchalamarla@...ium.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: Increase the max granular size

On Thu, Nov 05, 2015 at 08:45:08PM +0900, Joonsoo Kim wrote:
> 2015-11-05 19:32 GMT+09:00 Catalin Marinas <catalin.marinas@....com>:
> > On ARM we have a notion of cache writeback granule (CWG) which tells us
> > "the maximum size of memory that can be overwritten as a result of the
> > eviction of a cache entry that has had a memory location in it
> > modified". What we actually needed was ARCH_DMA_MINALIGN to be 128
> > (currently defined to the L1_CACHE_BYTES value). However, this wouldn't
> > have fixed the KMALLOC_MIN_SIZE, unless we somehow generate different
> > kmalloc_caches[] and kmalloc_dma_caches[] and probably introduce a
> > size_dma_index[].
> 
> If we create separate kmalloc caches for dma, can we apply this alignment
> requirement only to dma caches? I guess some memory allocation request
> that will be used for DMA operation doesn't specify GFP_DMA because
> it doesn't want the memory from ZONE_DMA. In this case, we should apply
> dma alignment requirement to all types of caches.

I think you are right. While something like swiotlb (through the
streaming DMA API) could do bounce buffering and allocate one from
ZONE_DMA, this is not guaranteed if the buffer physical address happens
to match the dma_mask. Similarly with an IOMMU, no bouncing happens but
the alignment is still required.

> If it isn't possible, is there another way to reduce memory waste due to
> increase of dma alignment requirement in arm64?

I first need to see how significant the impact is (especially for
embedded/mobiles platforms).

An alternative is to leave L1_CACHE_BYTES to 64 by default but warn if
the CWG is 128 on systems with non-coherent DMA (and hope that we won't
have any). It's not really fix, rather an assumption. Anyway I would
very much like the same kernel image for all platforms and no Kconfig
entry for the cache line size but if the waste is significant, we may
add one for some specific builds (like a mobile phone; or such vendors
could patch the kernel themselves).

-- 
Catalin
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