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

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Date:	Wed, 7 Apr 2010 17:11:25 +0200
From:	Daniel Mack <daniel@...aq.de>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	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, Apr 07, 2010 at 10:59:47AM -0400, Alan Stern wrote:
> On Wed, 7 Apr 2010, Daniel Mack wrote:
> > Depending on the condition of the memory management, things might work
> > or not, and especially right after a reboot, there's a better chance to
> > get lower memory.
> > 
> > The fix is to use usb_buffer_alloc() for that purpose which ensures
> > memory that is suitable for DMA. And on x86_64, this also means that the
> > upper 32 bits of the address returned are all 0's.
> 
> That is not a good fix.  usb_buffer_alloc() provides coherent memory, 
> which is not what we want.  I believe the correct fix is to specify the 
> GFP_DMA32 flag in the kzalloc() call.
> 
> Of course, some EHCI hardware _is_ capable of using 64-bit addresses.  
> But not all, and other controller types aren't.  In principle we could
> create a new allocation routine, which would take a pointer to the USB
> bus as an additional argument and use it to decide whether the memory 
> needs to lie below 4 GB.  I'm not sure adding this extra complexity 
> would be worthwhile.

Well, I thought this is exactly what the usb_buffer_alloc() abstraction
functions are there for. We already pass a pointer to a struct
usb_device, so the routine knows which host controller it operates on.
So we can safely dispatch all the magic inside this function, no?

If not, I would rather introduce a new function than adding GFP_ flags
to all existing drivers.

I vote for a clean solution, a fixup of existing implementations and
a clear note about how to allocate buffers for USB drivers. I believe
faulty allocations of this kind can explain quite a lot of problems on
x86_64 machines.

> > If what I've stated is true, there are quite some more drivers affected
> > by this issue.
> 
> Practically every USB driver, I should think.  And maybe a lot of 
> non-USB drivers too.

I found many such problems in drivers/media/{dvb,video},
drivers/usb/serial, sound/usb and even in the USB core. If we agree on
how to fix that nicely, I can take some of them.

Daniel
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