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SubjectRe: __get_free_pages()
On Thu, May 28, 1998 at 08:01:41PM -0400, James Mastros wrote:

> On Thu, 28 May 1998 ralf@uni-koblenz.de wrote:
> > Even that won't solve the problems with all the different architecture
> > aspects. Some Wrec^H^H^H^HDeskstation machines allocate DMA capable
> > memory for their ISA/VLB peripherals from a separate pool of memory which
> > is only intended for DMA but not for usage as ordinary system memory.
>
> It would require (AFAIK -- please, sombody correct me) some rather
> fundemental changes to everything that touches anything that does DMA to
> use this memory. (Indeed, I fail to see the point of this memory -- if
> you can't access it except by DMA, how can you put somthing there to be
> DMAed without DMAing twice?)

I said it's not being used as part of the normal system memory, that is
user pages are not being allocated from this special memory area called
``DMA Cache''. The ``DMA Cache'' is nothing but a 512kb or 2mb SRAM which
is connected to the host system via a 32 bit bus. The performance limit
imposed by the 32 bit bus, the limited size and fragmentation issues
which may be very serious in such a small area are why it isn't
recommendable as ordinary system memory. In theory it's useable however.
From the ISA bus side the DMA Cache is visible at bus address zero
and up; ISA peripherals cannot access other memory than the DMA Cache.

The mentioned box was designed to run NT, so software compatibility wasn't
a real problem. I suppose design idea of the DMA Cache was to decouple the
actual system bus (controlled by some standard Opti chipset) from the
ISA bus for better performance.

The point is DMA-able memory is not always a subset of the main memory,
it might come from a disjunct pool. The Amiga people can tell more
stories about different types of memory but they don't share many drivers
with the rest of the world. Just one of the things for 2.3 ...

Ralf

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