Re: DMA debug trace pointing to rtl8187

Michael Buesch
2009-05-09T20:21:26Z

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.