linux-kernel - Re: USB transfer_buffer allocations on 64bit systems

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Message-ID: <Pine.LNX.4.44L0.1004071149140.1779-100000@iolanthe.rowland.org>
Date:	Wed, 7 Apr 2010 11:55:19 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Greg KH <greg@...ah.com>
cc:	Daniel Mack <daniel@...aq.de>, <linux-kernel@...r.kernel.org>,
	Pedro Ribeiro <pedrib@...il.com>, <akpm@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>, <alsa-devel@...a-project.org>,
	<linux-usb@...r.kernel.org>
Subject: Re: USB transfer_buffer allocations on 64bit systems

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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