DMA Engine vs. DMA Transfer API

DMA Engine vs. DMA Transfer API

[Clifford Wolf]

Hi Shannon, hi lkml,

I'm writing a driver for the built-in DMA controller of the Freescale
MPC8349E PowerPC based microcontroller. Unfortunately the DMA-Engine
API seams to be the totally wrong thing for that since this DMA controller
has many features which are not available thru dmaengine afaics, such as:

 * Scatter-Gather on source and/or destination
 * Fifo modes for 8-, 16- and 32-bit (on source and dest)
 * Snooping mode, Multiple direct PCI read modes, ...

Also this DMA controller has only 4 channels (I do not know how many
channels I/OAT has). So I needed a spooling subsystem so many drivers
can easily share the available dma channels. The api also supports
more than one backend device and automatically finds out which backend
device fits a dma request best. I've named the entire thing dmatransfer
api. A current devel snapshot can be found at:

	http://www.clifford.at/priv/dmatransfer-20070711.diff

(I am on vacation right now and don't have access to my test hardware. So
the entire thing is 'frozen' right now until the second august week when
I'm back in the office.)

The smart infrasturcture adds some overhead: In a testcase copying 1GB of
data from memory to memory in 64kB chunks (300MHz DDR SDRAM, 500MHz PowerPC
CPU) this overhead is about 0.25% over controlling the dma controller
directly without my framework.

Afaics there is only one user of DMA Engine framework right now and only
one driver: The network stack.

any chances that we can migrate I/OAT to my more flexible DMA Engine
framework and modify the network stack to use dmatransfer instead of
dmaengine or in any other way integrate the APIs?

yours,
 - clifford

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RE: DMA Engine vs. DMA Transfer API

[Nelson, Shannon]

Clifford Wolf [mailto:clifford@clifford.at] 
>
>Hi Shannon, hi lkml,

Hi Clifford,

>I'm writing a driver for the built-in DMA controller of the Freescale
>MPC8349E PowerPC based microcontroller. Unfortunately the DMA-Engine
>API seams to be the totally wrong thing for that since this 
>DMA controller
>has many features which are not available thru dmaengine 
>[...]

This has been one of the biggest issues with the original dmaengine
interface - it didn't play well with others.  Since then a few folks
have taken a couple shots at it, but Dan Williams recently has done it
best in his changes aimed at rolling in his RAID accelleration engine.
Most of what you are looking for is already available - see the recent
changes in the dmaengine that allow for multiple DMA clients, multiple
DMA engines, and a method for selecting DMA services based upon the
capabilities needed.

If these changes still don't quite fit your need, let us know and we can
probably help you work it out.

Now, you go back to your vacation and I'll go back to my quiet Sunday
afternoon.

sln
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Mr. Shannon Nelson                 LAN Access Division, Intel Corp.
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