Re: [PATCH 0/6] get rid of on-stack dma buffers

[Florian Mickler <florian@mickler.org>]

On Mon, 21 Mar 2011 15:26:43 -0400
Andy Walls <awalls@md.metrocast.net> wrote:

> Florian Mickler <florian@mickler.org> wrote:
> 
> >Hi all!
> >
> >These patches get rid of on-stack dma buffers for some of the dvb-usb
> >drivers. 
> >I do not own the hardware, so these are only compile tested. I would 
> >appreciate testing and review.
> >They were previously sent to the list, but some error on my side
> >prevented (some of?) them from beeing delivered to all parties (the
> >lists).
> >
> >These changes are motivated by 
> >https://bugzilla.kernel.org/show_bug.cgi?id=15977 .
> >
> >The patches which got tested already were submitted to Mauro (and
> >lkml/linux-media) yesterday seperately. Those fix this same issue for
> >ec168,
> >ce6230, au6610 and lmedm04. 
> >
> >A fix for vp702x has been submitted seperately for review on the list.
> >I have
> >similiar fixes like the vp702x-fix for dib0700 (overlooked some
> >on-stack
> >buffers in there in my original submission as well) and gp8psk, but I
> >am
> >holding them back 'till I got time to recheck those and getting some
> >feedback
> >on vp702x.
> >
> >Please review and test.
> >
> >Regards,
> >Flo
> >
> >Florian Mickler (6):
> >  [media] a800: get rid of on-stack dma buffers
> >  [media v2] vp7045: get rid of on-stack dma buffers
> >  [media] friio: get rid of on-stack dma buffers
> >  [media] dw2102: get rid of on-stack dma buffer
> >  [media] m920x: get rid of on-stack dma buffers
> >  [media] opera1: get rid of on-stack dma buffer
> >
> > drivers/media/dvb/dvb-usb/a800.c   |   17 ++++++++++---
> > drivers/media/dvb/dvb-usb/dw2102.c |   10 ++++++-
> > drivers/media/dvb/dvb-usb/friio.c  |   23 ++++++++++++++---
> > drivers/media/dvb/dvb-usb/m920x.c  |   33 ++++++++++++++++--------
> > drivers/media/dvb/dvb-usb/opera1.c |   31 +++++++++++++++--------
> >drivers/media/dvb/dvb-usb/vp7045.c |   47
> >++++++++++++++++++++++++++----------
> > 6 files changed, 116 insertions(+), 45 deletions(-)
> >
> >-- 
> >1.7.4.1
> >
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> 

> Florian,
> 
> For all of these, what happens when the USB call times out and you kfree() the buffer?  Can the USB DMA actually complete after this kfree(), possibly corrupting space that has been reallocated off the heap, since the kfree()?
> 
> This is the scenario for which I assume allocating off the stack is bad.  
> 
> Do these changes simply make corruption less noticable since heap gets corrupted vs stack?
> 
> Regards,
> Andy

To be blunt, I'm not shure I fully understand the requirements myself.
But as far as I grasped it, the main problem is that we need memory
which the processor can see as soon as the device has scribbled upon
it. (think caches and the like)

Somewhere down the line, the buffer to usb_control_msg get's to be
a parameter to dma_map_single which is described as part of
the DMA API in Documentation/DMA-API.txt 

The main point I filter out from that is that the memory has to begin
exactly at a cache line boundary... 

I guess (not verified), that the dma api takes sufficient precautions
to abort the dma transfer if a timeout happens.  So freeing _should_
not be an issue. (At least, I would expect big fat warnings everywhere
if that were the case)

I cc'd some people that hopefully will correct me if I'm wrong...

regards,
Flo