Subject: Re: [PATCH 1/4] Add DMA sector to Documentation/powerpc/booting-without-of.txt file.

From: Scott Wood <[email-masked]>
Date: Thu, 12 Jul 2007 14:10:04 -0500

Phil Terry wrote:
> Say I'm writing a sound card, video card, widget card, etc., driver.
> 
> My driver gets loaded by virtue of detecting the card/device (via of,
> pci, usb, platform, whatever bus mechanisms).
> 
> My driver would benefit from using a generic DMA device so it uses the
> internal api to look for one and uses the API of that device to request
> a channel for its use. I don't care what channel I get, I don't need a
> fixed, reserved channel of the DMA device as specified in the dts I just
> need an unused channel. If no channels are available or I'm in a system
> with no generic DMA service available I can either fall back to
> processor copying or refuse to load.

The audio device example was not about using the DMA engine for generic 
memory copying; it was about an audio device that is hardwired to a 
certain DMA channel.  There is no way to do audio without using that 
specific channel.

> If the dts/of/boot loader tells the kernel its a fsl soc then it knows
> how to work out which one and what level, and therefore knows what
> devices, such as the DMA device are present.

All you'd be doing then is moving the device trees into the kernel.  The 
dtc syntax is a convenient way of expressing the information that has to 
live *somewhere*.

> Quite why we are using a 20 year old spec, which was never finished, and
> ceased to be a formal spec 10 years ago as the "new" way forward is a
> puzzle to me as well.

Probably because there was already code there to support it. :-P

It's not that bad in most respects, though unlike some, I don't think we 
need to stick dogmatically to the exact way that Open Firmware(tm) did 
everything.

-Scott