From: Michael Buesch <mb@bu3sch.de>
To: Greg KH <greg@kroah.com>
Subject: Re: DMA debug trace pointing to rtl8187
Date: Sat, 9 May 2009 22:19:39 +0200
Cc: Larry Finger <Larry.Finger@lwfinger.net>,
	Eric Valette <eric.valette@free.fr>,
	FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>,
	"John W. Linville" <linville@tuxdriver.com>,
	linux-wireless@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-usb@vger.kernel.org, Hin-Tak Leung <hintak.leung@gmail.com>
References: <49FDB9F8.3080400@free.fr> <4A05BD77.9020003@lwfinger.net> <20090509192959.GA1842@kroah.com>
In-Reply-To: <20090509192959.GA1842@kroah.com>
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On Saturday 09 May 2009 21:29:59 Greg KH wrote:
> On Sat, May 09, 2009 at 12:29:27PM -0500, Larry Finger wrote:
> > I think there is a second problem that John's fix does not treat. Although the
> > buffer is removed from the stack, there is no assurance that the buffer obtained
> > with kmalloc() is reachable by DMA. This case will be triggered if the USB
> > adapter does 32-bit DMA and the system has more than 4 GB RAM.

In practice this does not hit, because such systems' kmalloc does not return
memory above 4G (i386) _or_ the DMA mapping functions take care of bounce buffering
or I/O-remapping (should be true for all other arches).
So if the device is able to do DMA with addresses >=32bit it should be fine, provided
it correctly sets the DMA mask.

> Memory returned by kmalloc will always be able to be DMAable.  If not,
> we have lots of problems :)

True for sane devices.
False for devices like Broadcom HND-DMA, which should only be used to slap thy hw engineers.

-- 
Greetings, Michael.