On Mon, 2015-03-16 at 22:37 +0100, Jiri Kosina wrote:
> On Mon, 16 Mar 2015, Pavel Machek wrote:
>
> > > > Oliver Neukum <oneu...@suse.de> wrote:
> > > >
> > > > > > + ret = usb_interrupt_msg(dev, usb_sndintpipe(dev, 0x02),
> > > > > > + buf2, sizeof(buf2),
> > > > > > + &transfered, USB_CTRL_SET_TIMEOUT);
> > > > >
> > > > > You cannot do this. Even for a single byte DMA on the stack is
> > > > > wrong. Not on all architectures it works at all and you violate
> > > > > the DMA constrainsts. You must use kmalloc().
> > > >
> > > > Hi Oliver,
> > > >
> > > > Does this still apply when using hid_hw_output_report?
> > >
> > > Yes. For USB devices hid_hw_output_report() goes to
> > > usbhid_output_report(). That goes to usb_interrupt_msg(),
> > > which passes the buffer pointer. It will then be mapped
> > > for DMA. You must not do that on the stack.
> >
> > Should we have some kind of runtime test for this ...? Because this is
> > very very easy to get wrong... and I bet we do get it wrong at > 1
> > place...
>
> Are you sure CONFIG_DMA_API_DEBUG wouldn't warn here?
As far as I can tell, it will not warn. The problem is not in the
mapping itself. That is usually legitimate. The problem arises
because the buffer doesn't have a cacheline of its own. Thus the
memory corruption happens after the IO operation has started.
Regards
Oliver
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