Am Montag, 2. Mai 2005 03:38 schrieb David Brownell:
> It might well be simpler to just pin whatever (aligned) buffers
> have been passed, ensure they're properly flushed, and then just
> DMA to/from those pages without requiring special DMA mappings
> to be set up first, and without needing special new usbfs calls.
How would you make sure the buffers passed are DMA-able?
Also, how would you make sure they can be flushed independently?
You avoid that trouble by providing multiples of PAGE_SIZE through
mmap. Plus, you can easily make available the _sg_ API to user space
this way.
> I was looking at that sort of stuff a while back, in conjunction
> with seeing how the AIO stuff might replace the rather funky
> USB-specific AIO-ish stuff in usbfs. A lot of the relevant
> infrastructure is already in place.
Yes, ideally we had files for each endpoint and use AIO. We haven't,
though.
> As an example, we now have AIO support in "gadgetfs", which I've
> suggested should be the basic model to follow when rewriting
> "usbfs". With a one-to-one mapping between URBs (or usb_requests)
> and kiocbs, an incremental development step might be as simple as
> just adding an ioctl to usbfs to return a new AIO-capable file
> handle for a given endpoint, then using normal AIO calls on that
> to reuse some of the existing zerocopy work ...
Good idea.
[..]
> The key point to draw from that is that all this zerocopy stuff
> can (and should!!) be done at layers above usbcore. If the
> layer above -- "usbfs", "usbfs2", or even "usbfs 1.5" -- passes
> URBs with DMA mappings already established, all the nasty/fragile
> usbcore stuff can be left alone. And even "usbfs" could mostly
> be left alone.
Yes.
Regards
Oliver
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