Re: [BK PATCH] USB update for 2.6.3

[Hollis Blanchard]

On Feb 20, 2004, at 9:15 AM, Linus Torvalds wrote:
> On Thu, 19 Feb 2004, David S. Miller wrote:
> > On Fri, 20 Feb 2004 18:10:41 +1100
> > Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > > Hrm... so if the USB device drivers are actually doing the dma 
> > > mapping
> > > themselves, it make sense for them to pass their own struct device, 
> > > no ?
> >
> > That's right, at least that was the idea.
>
> No. That would be _fundamentally_ wrong.
>
> There's no way a USB device can do DMA in the first place. It has no 
> DMA
> controller, and no way to read/write memory except through the USB 
> host.
>
> So it is the host - and only the host - that matters. Anything else is 
> a
> bug.

Sure. So dma-mapping.h does this:
	int dma_supported(struct device *dev, u64 mask) {
		return device->bus->dma_supported(dev, u64 mask);
	}

And USB, when it creates its bus_type, does this:
	int usb_dma_supported(struct device *dev, u64 mask) {
		usb_dev *usbdev = to_usb_device(dev);
		return usbdev->root_hub->controller->bus->dma_supported(controller, 
u64 mask)
	}
And of then PCI has:
	int pci_dma_supported(struct device *dev, u64 mask) {
		pci_dev *pcidev = to_pci_dev(dev, u64 mask);
		...
	}

Then a USB driver uses its own usb_device->dev and it all ends up back 
at the PCI bus.

For PCI devices of course, device->bus->dma_supported() *is* 
pci_dma_supported(), so there's no middleman (as USB is above).

--
Hollis Blanchard
IBM Linux Technology Center

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