Subject: Slight optimization for bus_dma(9) API.
To: None <>
From: Jason R Thorpe <>
List: tech-kern
Date: 07/18/2001 23:11:21
Hi folks...

I would like to add 2 flags to the bus_dma(9) API, to be used with
the bus_dmamap_load*() family of routines.

	BUS_DMA_INBOUND		Transfer is BUS->RAM direction.

	BUS_DMA_OUTBOUND	Transfer is RAM->BUS direction.

These flags would be mutually-exclusive.  If you specify one, it
specifies the direction that the transfer will perform.  It is
meant to allow machine-dependent code to make optimizations based
on the direction.  If neither flag is specified, no directional
optimization will be performed.

I want this primarily for the Alpha port.  For the Alpha SGMAP
(DMA address MMU), for the RAM->BUS (OUTBOUND) direction, the
PCI controller will pre-fetch the next page of memory if the
current DMA address comes within a certain distance of the next
page.  This is meant to prevent the DMA pipeline from stalling.

Unfortunately, if you reach the prefetch threshold on the *last
page* of the DMA transfer, then you need to have an extra PTE
after the end of the buffer to map a "spill page".  Otherwise,
you will get a machine check when the PCI controller attempts
to fetch a page that is not mapped by the DMA MMU.

If I know which the direction is going to be used when the map
is loaded, I can avoid allocating the spill page in certain
cases, for example, for all in-bound traffic :-)

It was also suggested to me that you could cache a pointer to
a dmamap_sync routine that performs fewer tests, as well.

Note that machine dependent code can choose to ignore the flags,
and drivers can choose not to use them.  They are merely to
allow for an optimization.

If there are no objections, I'll go ahead and add this before the
end of the week.

        -- Jason R. Thorpe <>