Re: Split bulk transfers

On Wednesday 09 May 2007, Laurent Pinchart wrote:

> I wasn't aware of those functions. Just to make sure I get this right, am I 
> right to say that URB buffers should be allocated as contiguous cache 
> coherent physical memory (using usb_buffer_alloc, which in turns call 
> dma_alloc_coherent for big DMAable buffers)

Not many applications need cache-coherent buffers.
Few of them involve "big" transfers.


> or as normal memory using scatter-gather routines ?

Likewise not many stacks hand you scatterlists.  If you
get a scatterlist, there's no real option about how to
do that I/O:  either dma map yourself, or let usbcore
handle it for you (using the usb_sg_* calls)


> I currently allocate big bulk buffers using usb_buffer_alloc, which puts a lot 
> of pressure on the memory allocator.

Bad answer, unless you really need that kind of buffer.
>From what you've said, you don't need them, and should
not use those allocator calls.


> Using scatter-gather mappings would  
> help. However, if I'm not mistaken, the usb_sg_* routines are designed for 
> synchronous use (with usb_sg_wait), and thus can't be used for streaming 
> data.

Right.  Scatterlists come from the block layer, which
doesn't stream.

- Dave



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