linux-kernel - Re: DMA debug trace pointing to rtl8187

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Date:	Sat, 9 May 2009 22:19:39 +0200
From:	Michael Buesch <mb@...sch.de>
To:	Greg KH <greg@...ah.com>
Cc:	Larry Finger <Larry.Finger@...inger.net>,
	Eric Valette <eric.valette@...e.fr>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	"John W. Linville" <linville@...driver.com>,
	linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org, Hin-Tak Leung <hintak.leung@...il.com>
Subject: Re: DMA debug trace pointing to rtl8187

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.
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