Date: Thu, 23 Aug 2012 10:59:59 -0500 From: Alan Cox <alc@rice.edu> To: John Baldwin <jhb@freebsd.org> Cc: svn-src-head@freebsd.org, Oleksandr Tymoshenko <gonzo@freebsd.org>, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r239597 - head/sys/arm/arm Message-ID: <5036537F.2060801@rice.edu> In-Reply-To: <201208230746.57495.jhb@freebsd.org> References: <201208222248.q7MMmojL011382@svn.freebsd.org> <201208230746.57495.jhb@freebsd.org>
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On 08/23/2012 06:46, John Baldwin wrote: > On Wednesday, August 22, 2012 6:48:50 pm Oleksandr Tymoshenko wrote: >> Author: gonzo >> Date: Wed Aug 22 22:48:50 2012 >> New Revision: 239597 >> URL: http://svn.freebsd.org/changeset/base/239597 >> >> Log: >> Do not change "cachable" attribute for DMA memory allocated with >> BUS_DMA_COHERENT attribute >> >> The minimum unit for changing "cachable" attribute is page, so call >> to pmap_change_attr effectively disable cache for all pages that newly >> allocated DMA memory region spans on. The problem is that general-purpose >> memory could reside on these pages too and disabling cache might affect >> performance. Moreover ldrex/strex operators raise Data Abort exception >> when accessing memory on page with "cachable" attribute off. >> >> BUS_DMA_COHERENT does nto require memory to be coherent. It just suggests >> to do best effort for reducing synchronization overhead. > Alternatively you could force an allocation with BUS_DMA_COHERENT to always > allocate at least one page. Also, it is probably better to match what I > recently did in x86 and use kmem_alloc_contig() or kmem_alloc_attr() for > allocations with a custom memory attribute rather than using > pmap_change_attr() directly. > The arm pmap doesn't have the necessary support for memory attributes. pmap_page_{get,set}_memattr() are just stubs. Alan
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