Re: Is usb_buffer_alloc a generic URB buffer allocator ? (was Split Bulk Transfers)

On Tue, 15 May 2007, Laurent Pinchart wrote:

> Hi everybody,
> 
> following the discussion about the split bulk transfers, Alan Stern and David 
> Brownell told me I shouldn't use usb_buffer_alloc as a generic purpose URB 
> buffer allocated. However, Documentation/usb/dma.txt contradicts this. Should 
> the documentation be fixed, or can/should usb_buffer_alloc be used to 
> allocate URB buffers ?

It's difficult to give general advice, since the performance will vary 
according to the platform and the usage.

If a buffer is going to be used once and then thrown away, I think 
there's not much advantage to usb_buffer_alloc().  Although on some odd 
platforms that may not be true, it should be true on the vast majority.

If a buffer is going to be used many times, then usb_buffer_alloc() 
might give improved performance.  Again, that depends on the platform 
and the type of memory accesses involved.

The tradeoffs are hard to assess because they involve a lot of 
variables: How responsive is the IOMMU, does the cache do 
bus-snooping, will there be many accesses to uncached memory, and so 
on.

Alan Stern


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