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

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Date:	Mon, 12 Apr 2010 19:15:07 +0200
From:	Daniel Mack <daniel@...aq.de>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Andi Kleen <andi@...stfloor.org>, Pedro Ribeiro <pedrib@...il.com>,
	linux-kernel@...r.kernel.org, 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 Mon, Apr 12, 2010 at 12:57:16PM -0400, Alan Stern wrote:
> On Mon, 12 Apr 2010, Andi Kleen wrote:
> > Hmm, thanks. But things must still go wrong somewhere, otherwise
> > the GFP_DMA32 wouldn't be needed?
> 
> Indeed, something must go wrong somewhere.  Since Daniel's patch fixed
> the problem by changing the buffer from a streaming mapping to a
> coherent mapping, it's logical to assume that bad DMA addresses have
> something to do with it.  But we don't really know for certain.

Given that - at least for non-64-aware host controllers - we want memory
<4GB anyway for USB transfers to avoid DMA bouncing buffers, maybe we
should just do that and fix the problem at this level? I already started
to implement usb_[mz]alloc() and use it in some USB drivers.

But even after all collected wisdom about memory management in this
thread, I'm still uncertain of how to get suitable memory. Using
dma_alloc_coherent() seems overdone as that type of memory is not
necessarily needed and might be a costly good on some platforms. And as
fas as I understand, kmalloc(GFP_DMA) does not avoid memory >4GB.

Can anyone explain which is the right way to go?

Daniel
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