Re: USB transfer_buffer allocations on 64bit systems

From: Takashi Iwai
Date: Wed Apr 07 2010 - 13:52:47 EST

At Wed, 7 Apr 2010 11:55:19 -0400 (EDT),
Alan Stern wrote:
> 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.

Yeah, also the area is aligned to kernel pages, and it may be much
bigger than the requested (power-of-two). If not needed, we should
avoid it.


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