Re: [patch/rfc 2.6.22-rc7] usb/dma doc updates

Am Montag, 2. Juli 2007 schrieb David Brownell:
> On Monday 02 July 2007, Oliver Neukum wrote:
> 
> > And please explain why we can operate efficiently with kmalloced
> > memory although we don't evaluate the controller's dma mask.
> 
> In what way does that relate to anything I updated?  Looks like
> you want me to introduce a new topic, which hasn't been raised
> before as a point of confusion ...

You are mentioning the DMA bounce buffers and therewith the dma mask:

+  For those specific cases, USB has primitives to allocate less expensive
+  memory.  They work like kmalloc and kfree versions that give you the right
   kind of addresses to store in urb->transfer_buffer and urb->transfer_dma,
   while guaranteeing that no hidden copies through DMA "bounce" buffers will
   slow things down.  You'd also set URB_NO_TRANSFER_DMA_MAP in

In other words, the first part of the text tells you that there is an API
that has benefits A & B. Later it says that it has a disadvantage that
usually outweighs benefit B. But benefit A is not mentioned.
This doesn't add up.

	Regards
		Oliver

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