Re: USB transfer_buffer allocations on 64bit systems

From: Alan Stern
Date: Wed Apr 07 2010 - 11:55:26 EST

On Wed, 7 Apr 2010, Greg KH wrote:

> Yeah, I really don't want to have to change every driver in different
> ways just depending on if someone thinks it is going to need to run on
> this wierd hardware.

It's not weird hardware, as far as I know. It's just a 64-bit system
with a 32-bit USB host controller.

(And remember, while there are 64-bit EHCI controllers, there are not
any 64-bit OHCI or UHCI controllers. So whenever somebody plugs a
full-speed or low-speed device into a 64-bit machine, they will face
this problem. It's like the old problem of ISA devices that could
only do DMA to addresses in the first 16 MB of memory -- what the
original GFP_DMA flag was intended for.)

> Alan, any objection to just using usb_buffer_alloc() for every driver?
> Or is that too much overhead?

I don't know what the overhead is. But usb_buffer_alloc() requires the
caller to keep track of the buffer's DMA address, so it's not a simple
plug-in replacement. In addition, the consistent memory that
usb_buffer_alloc() provides is a scarce resource on some platforms.

Writing new functions is the way to go.

Alan Stern

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