[uml-devel] Re: [linux-usb-devel] Infrastructure for using USB with user mode linux on 2.5.34, ARM questions

[David B. <da...@pa...>]

> I was looking for opinons on which way to approach the problem, I dont wan't
> to reinvent the wheel (especially as mine seem start out as a triangle)

I noticed that ... :)


> On the SLAB_DMA being smarter, that is good does it relate to those comments
> about some hardware only working with DMA in the 1st 1MiB of memory, less
> even then some ISA hardware.

Smarter for that particular platform.  In general, what the PCI DMA calls do
is arch-specific; in that one, it's slab code underneath.


> complexity and hits that axiom again, it is not good to duplicate code among
> the HCDs either, so I would have likely aimed for the hcd-pci.o file and
> suggest the fake-pci.o file with the kmalloc/NOP equivialents for ARMs OHCI,
> and leave uml-hcd without either.

UML will need _something_ to implement those usb_operations.


> 0) UML private fake pci support
> pro: easist, no changes to other subsystems, already present with patches I
> am using
> con: no code shareing, duplicated code in 3 places (mainline + ARM + UML)

This fits best with the "null pci_dev is platform-defined" policy that's
been the standard hack for a couple years now.  That "con" would IMO
be best addressed by moving DMA memory issues into the device model,
but then UML would _still_ need to have a solution to this problem.

> 2) reverting buffer.c etc.
> pro: it used to work, it might also simpilfy ARM
> con: I assume that there was a reason to put this in to begin with :)
> But what is that reason?

Kerneldoc for the usb_buffer_*() functions describes some reasons, like
elimination of buffer copies (for SA-1111 and others), reducing IOMMU
overhead (for SPARC and some others), and enabling better IOMMU mappings.

If it simplified ARM, it'd do so at the cost of ensuring every SA-1111
I/O involve copying through a DMA bounce buffer ... right now, drivers
can eliminate them by using usb_buffer_{alloc,free}().

- Dave