On Sun, 18 Aug 2013, Geert Uytterhoeven wrote:
> On Thu, Jul 11, 2013 at 1:12 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > On Wednesday 10 July 2013, Alan Stern wrote:
> >> This isn't right. There are USB host controllers that use PIO, not
> >> DMA. The HAS_DMA dependency should go with the controller driver, not
> >> the USB core.
> >>
> >> On the other hand, the USB core does call various routines like
> >> dma_unmap_single. It ought to be possible to compile these calls even
> >> when DMA isn't enabled. That is, they should be defined as do-nothing
> >> stubs.
> >
> > The asm-generic/dma-mapping-broken.h file intentionally causes link
> > errors, but that could be changed.
> >
> > The better approach in my mind would be to replace code like
> >
> >
> > if (hcd->self.uses_dma)
> >
> > with
> >
> > if (IS_ENABLED(CONFIG_HAS_DMA) && hcd->self.uses_dma) {
> >
> > which will reliably cause that reference to be omitted from object code,
> > but not stop giving link errors for drivers that actually require
> > DMA.
>
> This can be done for drivers/usb/core/hcd.c.
>
> But I'm a bit puzzled by drivers/usb/core/buffer.c. E.g.
>
> void *hcd_buffer_alloc(...)
> {
> ....
> /* some USB hosts just use PIO */
> if (!bus->controller->dma_mask &&
> !(hcd->driver->flags & HCD_LOCAL_MEM)) {
I don't remember the full story. You should check with the person who
added the HCD_LOCAL_MEM flag originally.
> So if DMA is not used (!hcd->self.uses_dma, i.e. bus->controller->dma_mask
> is zero), and HCD_LOCAL_MEM is set, we still end up calling dma_pool_alloc()?
>
> (Naively, I'm not so familiar with the USB code) I'd expect it to use
> kmalloc() instead?
Maybe what happens in this case is that some sort of hook causes the
allocation to be made from a special memory-mapped region on board the
controller.
Alan Stern
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