Re: [BK PATCH] USB update for 2.6.3

[Linus Torvalds]

On Fri, 20 Feb 2004, Hollis Blanchard wrote:
> 
> 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)
> 	}

So? The above has absolutely nothing to do with "dma_alloc_coherent()". 
Also, it is wrong. It is not necessarily guaranteed that the device that 
is the host controller ha sanythign to do with the "bus->dma_supported" 
thing.

The point is, that DMA is always _always_ done on the host controller. 
Trying to make things look any different is silly and wrong.

THE ABOVE CODE IS CRAP!

The correct thing to do is

	int usb_dma_supported(struct usb_dev *dev, u64 mask)
	{
		struct device *host = dev->root_hub->controller;
		return dma_supported(host);
	}

and anything else is FUNDAMENTALLY WRONG!

Imagine, for example, that the bus is a PCI bus, but the USB host 
controller has a bug in that it only supports 24-bit DMA. Asking for what 
the bus of the host controller supports is non-sensical, and has 
absolutely zero to do with that the actual host device supports.

See?

(That actual bug is totally irrelevant, though. The _fundamnetal_ bug is
in your way of thinking. For one thing, if you have a function called
"usb_dma_xxx()", then it takes a _USB_ device, not a generic device.  
Because the function clearly doesn't even WORK with a generic device, it
only works with a "struct usb_dev". So don't "lie" about things like that
in your interfaces and confuse the issue).

			Linus
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