linux-kernel - Re: [PATCHv4 5/5] arm64: Add atomic pool for non-coherent and CMA allocations.

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Date:	Tue, 22 Jul 2014 16:51:04 -0700
From:	Laura Abbott <lauraa@...eaurora.org>
To:	Catalin Marinas <catalin.marinas@....com>,
	Arnd Bergmann <arnd@...db.de>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Will Deacon <Will.Deacon@....com>,
	David Riley <davidriley@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Ritesh Harjain <ritesh.harjani@...il.com>
Subject: Re: [PATCHv4 5/5] arm64: Add atomic pool for non-coherent and CMA
 allocations.

On 7/22/2014 2:03 PM, Catalin Marinas wrote:
> On Tue, Jul 22, 2014 at 07:06:44PM +0100, Arnd Bergmann wrote:
[...]
>>> +               if (!addr)
>>> +                       goto destroy_genpool;
>>> +
>>> +               memset(addr, 0, atomic_pool_size);
>>> +               __dma_flush_range(addr, addr + atomic_pool_size);
>>
>> It also seems weird to flush the cache on a virtual address of
>> an uncacheable mapping. Is that well-defined?
> 
> Yes. According to D5.8.1 (Data and unified caches), "if cache
> maintenance is performed on a memory location, the effect of that cache
> maintenance is visible to all aliases of that physical memory location.
> These properties are consistent with implementing all caches that can
> handle data accesses as Physically-indexed, physically-tagged (PIPT)
> caches".
> 

This was actually unintentional on my part. I'm going to clean this up
to flush via the existing cached mapping to make it clearer what's going
on.

>> In the CMA case, the
>> original mapping should already be uncached here, so you don't need
>> to flush it.
> 
> I don't think it is non-cacheable already, at least not for arm64 (CMA
> can be used on coherent architectures as well).
> 

Memory allocated via dma_alloc_from_contiguous is not guaranteed to be
uncached. On arm, we allocate the page of memory and the remap it as
appropriate.

>> In the alloc_pages() case, I think you need to unmap
>> the pages from the linear mapping instead.
> 
> Even if unmapped, it would not remove dirty cache lines (which are
> associated with physical addresses anyway). But we don't need to worry
> about unmapping anyway, see above (that's unless we find some
> architecture implementation where having such cacheable/non-cacheable
> aliases is not efficient enough, the efficiency is not guaranteed by the
> ARM ARM, just the correct behaviour).
> 

Let's hope that never happens.

Thanks,
Laura

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