Re: API change for bus_dma

From: Andrew Gallatin (gallatin_at_cs.duke.edu)
Date: 06/30/03 

Date: Mon, 30 Jun 2003 10:03:46 -0400 (EDT)
To: Terry Lambert <tlambert2@mindspring.com>

Terry Lambert writes:
> Andrew Gallatin wrote:
> > Most sparc's have 2 different sorts of DMA modes. One is cache
> > coherent (aka DDI_DMA_CONSISTENT) -- this is what we all know and love
> > from PC, alphas, macs, etc.
>
> "contiguous"
>
>
> > The other mode (DDI_DMA_STREAMING) allows non cache coherent DMA.
> > This requires you to call ddi_dma_sync() between your last touch of
> > the data and you starting a DMA read from a device. And vice-versa
> > for a DMA write.
>
> "scatter/gather"
>
> > The reason people use DDI_DMA_STREAMING is because coherent DMA
> > bandwith tends to be abysmal on most sparcs.
>

The idea is that I want to establish a mapping that can be used many
times without any driver or kernel attention. I don't want to do
anything in terms of a system call, or interrupt, etc, to sync the
cache with the state of the DMA'ed page before a DMA read or after a DMA
write.

For example, scatter gather mapping on alphas (if FreeBSD supported it
for PCI devices) would be fine with me since its cache-coherent and
doesn't require any ddi_dma_sync() operations.

Drew

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