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SubjectRe: Help with mapping memory into kernel space?
On Wed, 18 Aug 2004, Daniel Gryniewicz wrote:

> On Tue, 2004-08-17 at 22:39 -0700, Brian McGrew wrote:
> > Good day All:
> >
> <snip>
> > The overall problem is that the more system memory we install,
> > the fewer IBB's we can use. For instance, 256MB lets us use
> > four IBB's; 512MB lets us use three IBB's and so on. Basicly,
> > the kernel blows up trying to map memory. Each IBB has two
> > banks of 64MB of RAM on them which we try and memmap to system
> > memory for speed of addressing. So essentaily, we're sending
> > out four camera systems with only 256MB of memory which is only
> > about one quarter of what we need.
> On x86, the kernel has 1 GiB of address space. Try the 2G/2G split
> patches, or the 4G/4G patches, either one of which should increase your
> kernel address space enough to map both memory and your buffers.
> Daniel

> out four camera systems with only 256MB of memory which is only

You can use as much memory as you can afford if the designer of
this system would only understand that you can't fit 4 GB of
application RAM into 4 GB of address-space and leave anything
available for the system.

It needs to be done like this:

Other | N Mbyte | Other address space
address | window | ..................
| Page register |
| Page 0 |
| Page 1 |
Page N, etc.

You make a small window in the available address-space, and you
map your RAM into that address-space with a page register. You
can access much, much more RAM than available address-space,
and with things like video and pictures, the occasional switching
of a page-register adds practically no overhead.

Dick Johnson
Penguin : Linux version 2.4.26 on an i686 machine (5570.56 BogoMips).
Note 96.31% of all statistics are fiction.

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