Re: [PATCH 0/3] RFC: addition to DMA API

[Will Deacon]

Hi Russell,

On Thu, Sep 01, 2011 at 05:04:29PM +0100, Russell King - ARM Linux wrote:
> DMA coherent memory on ARM is implemented on ARMv5 and below by using
> 'noncacheable nonbufferable' memory.  There is no weak memory model to
> worry about, and this memory type is seen as 'strongly ordered' - the
> CPU stalls until the read or write has completed.  So no problem there.
> 
> On ARMv6 and above, the attributes change:
> 
> 1. Memory type: [Normal, Device, Strongly ordered]
>    All mappings of a physical address space are absolutely required to be
>    of the same memory type, otherwise the result is unpredictable.  There
>    is no mitigation against this.
> 
> 2. For "normal memory", a variety of options are available to adjust the
>    hints to the cache and memory subsystem - the options here are
>    [Non-cacheable, write-back write alloc, write-through non-write alloc,
>     write-back, non-write alloc.]
> 
>    Strictly to the ARM ARM, all mappings must, again, have the same
>    attributes to avoid unpredictable behaviour.  There is a _temporary_
>    architectural relaxation of this requirement provided certain conditions
>    are met - which may become permanent.

This looks set to appear in revision C of the ARM ARM.

> It _is_ possible that "unpredictable" means that we may hit cache lines in
> the [VP]IPT cache via the non-cacheable mapping which have been created
> by speculative loads via the cacheable mapping - and this is something
> that has been worrying me for a long time.

Whilst this can happen, this will only cause problems for reads performed
by the CPU (as these may hit a line speculatively loaded via the cacheable
alias). Setting bit 22 in the auxillary control register gets arounds this:

http://www.arm.linux.org.uk/developer/patches/viewpatch.php?id=6529/1

Given that I believe our coherent DMA memory is `cacheable, bufferable, do
not allocate' in terms of AXI attributes, then writes will go straight to
the write buffer on the PL310.

> So, in summary what I'm saying is that _in theory_ our DMA coherent memory
> on ARMv6+ should have nothing more than write buffering to contend with,
> but that doesn't stop this being the first real concrete report proving
> that what I've been going on about regarding the architectural requirements
> over the last few years is actually very real and valid.

I don't think what we're seeing in this case is caused by mismatched memory
attributes, especially as passing `nosmp' on the command-line makes the
performance issue disappear.

Will
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