Re: Accessing memory remote across the bus in the same way as local memory

[James Bottomley <James.Bottomley@SteelEye.com>]

On Fri, 2004-06-18 at 23:05, Paul Mackerras wrote:
> Not on iSeries machines (ppc64 machines running OS/400).  There we
> have to do a hypervisor call for every access to PCI memory or I/O
> space, unfortunately.  Fortunately people don't tend to put arbitrary
> PCI adaptors in iSeries boxes, and they don't run X servers.  Linux
> partitions on iSeries machines are mostly used for things like
> firewalls and web servers.

Heh, I guessed there'd be one architecture that couldn't do this.

> What did you want to be able to do this for?  It probably isn't the
> end of the world if whatever you want to do with this doesn't work on
> iSeries.

The basic issue is can we declare memory that resides on a device chip
(or on a memory controller behind a PCI bridge) to the system so that it
can be used almost identically to real memory (just with a bunch of
restrictions).

The request is coming from an ARM developer who has an OHCI chip with 32
on chip sram.  He wants dma_alloc_coherent() to hand out the on-chip
memory so that he doesn't have to modify the generic OHCI driver to be
able to use a separate memory area.

However, most modern devices seem to come with on-board memory for
descriptors, ring buffers and the like (mainly for efficiency since the
device can access them without having to do a bus to main memory
transaction), so I was wondering if allocating from this on-board pool
could be folded into the DMA API so that dma_alloc_coherent() could
manage it for every such device.

James