[PATCH 01/10] ARM: change ARM_DMA_ZONE_SIZE into a variable

[John Linn]

> -----Original Message-----
> From: Russell King - ARM Linux [mailto:linux at arm.linux.org.uk]
> Sent: Friday, July 08, 2011 3:00 AM
> To: John Linn
> Cc: Arnd Bergmann; linux-arm-kernel at lists.infradead.org; Nicolas
Pitre;
> grant.likely at secretlab.ca; Barry Song
> Subject: Re: [PATCH 01/10] ARM: change ARM_DMA_ZONE_SIZE into a
> variable
> 
> On Thu, Jul 07, 2011 at 11:08:38AM -0600, John Linn wrote:
> > If I'm following this right....
> >
> > We have a hole at the start of physical memory (when it's at 0) so
> that
> > DMA can't DMA into it.  Sounds like that won't work for us?
> 
> That depends what you mean by that.
> 
> If you're saying that your physical RAM starts at some non-zero
> address,
> then that isn't a problem.  We have lots of ARM platforms where that
is
> true.
> 
> If you're saying that the first N MB of RAM can't be DMA'd to, then
> that
> is a problem as the kernel has no way to be told about that
> restriction.
> The best we can manage is to avoid freeing that memory into the normal
> allocation pools.

Yes that is what I'm saying. Catalin and I had some conversation about
this
previously (my apologies if I just didn't get it Catalin).

> 
> We do have a very small number of platforms where that's true, but we
> 'work around' that by having the kernel occupy that space.  If N MB is
> less than sizeof(kernel) + 32K then you haven't lost much.

It's only at addresses 0 - 512K that's not DMA-able.  I am using a DMA
zone (with a hole at the front) to avoid this problem right now but
maybe I'm not really understanding it and it's not working correctly.
Testing looks good with the DMA zone.

I didn't like the other solution since we had to add another exception
where the kernel frees it's memory (not to free it).  But maybe this is
really the right solution as Catalin had tried to explain to me.

Thanks for the input,
John


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