Re: [PATCH][RFC] USB: zerocopy support for usbfs

[Alan Stern]

On Wed, 2 Jul 2014, Peter Stuge wrote:

> > The kernel doesn't support scatter-gather for control transfers, only 
> > bulk.
> 
> That could possibly change, right, and then it would be nice to have
> zerocopy for free there as well?

No.  ohci-hcd doesn't support control transfers larger than 4 KB
anyway, and ehci-hcd doesn't support control transfers larger than 16 
KB.  This is not likely to change.  Besides, a control transfer can 
never be larger than 64 KB, and throughput isn't an issue for them.

> > > Indeed I think userspace wants to be involved in choosing memory also
> > > with bulk, in order to ensure that zerocopy will always work when
> > > userspace cares about that.
> > > 
> > > Is it enough to expose the DMA mask of the host controller?
> > 
> > It doesn't need to be exposed, since the mmap(2) call would be handled
> > by the kernel's USB stack (and besides, the user program can't request
> > that the mapped memory be located in any particular physical address
> > region).
> 
> Since alignment isn't the only issue I don't think there's a way to
> avoid it. I was just hoping to be able to avoid allocating zerocopy
> buffers with mmap().

How else can you guarantee that a buffer is located in the first 4 GB
of memory?  Even on a 32-bit system (if the computer has more than 4 GB 
of RAM)?

Alan Stern

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