Re: [Ksummit-2009-discuss] Representing Embedded Architectures at the Kernel Summit

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 3 Jun 2009 18:09:25 +0100
Russell King <rmk@arm.linux.org.uk> wrote:

> In
> fact, on ARM the DMA mask is exactly that - it's a 100% proper mask.  It's
> not a bunch of zeros in the MSB followed by a bunch of ones down to the
> LSB.  It can be a bunch of ones, a bunch of zeros, followed by a bunch of
> ones.
> 
> The way we occasionally have to deal with this is to trial an allocation,
> see if the physical address fits, if not free the page and try again with
> GFP_DMA set.

A couple of times I've suggested that we have the ability to allocate
one zone per address bit, so a 32-bit machine with 4k pages would end
up having 20 zones.  Then, your funny DMA mask can be directly passed
into the page allocator as a zone mask and voila, I think.

> There's many stories I've heard on what is supposed to take care of the
> coherency that I now just close my ears to the problem and chant "it
> doesn't exist, people aren't seeing it, mainline folk just don't give
> a damn".  Really.  It is a problem on _some_ ARM devices and has been
> for several years now, and I've 100% given up caring about it.

I wasn't even aware that there was an issue here.  Please don't blame
"mainline folk" for something they weren't told about!