Re: [PATCH] firewire-ohci: work around oversized DMA reads on JMicron controllers

[Stefan Richter <stefanr@s5r6.in-berlin.de>]

On Nov 03 Clemens Ladisch wrote:
> Hector Martin wrote:
> > At least some JMicron controllers issue buggy oversized DMA reads when
> > fetching context descriptors, always fetching 0x20 bytes at once for
> > descriptors which are only 0x10 bytes long. This is often harmless, but
> > can cause page faults on modern systems with IOMMUs:
> >
> > DMAR: [DMA Read] Request device [05:00.0] fault addr fff56000 [fault reason 06] PTE Read access is not set
> > firewire_ohci 0000:05:00.0: DMA context IT0 has stopped, error code: evt_descriptor_read
> >
> > This works around the problem by always leaving 0x10 padding bytes at
> > the end of descriptor buffer pages, which should be harmless to do
> > unconditionally for controllers in case others have the same behavior.
> >
> > Signed-off-by: Hector Martin <marcan@marcan.st>  
> 
> Reviewed-by: Clemens Ladisch <clemens@ladisch.de>

Thanks Hector for the troubleshooting and for the patch.
Thanks Clemens for the review.

It's been a while since I last reviewed and tested kernel patches, and
also my main FireWire equipped PC is currently tied up in work for which
reboots aren't desirable.  But I am updating a long unused secondary
FireWire'd PC right now and give the patch some testing this week.  (This
one even has a JMicron controller, but not an IOMMU.)

> > ---
> >  drivers/firewire/ohci.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/firewire/ohci.c b/drivers/firewire/ohci.c
> > index 8bf89267dc25..d731b413cb2c 100644
> > --- a/drivers/firewire/ohci.c
> > +++ b/drivers/firewire/ohci.c
> > @@ -1130,7 +1130,13 @@ static int context_add_buffer(struct context *ctx)
> >  		return -ENOMEM;
> >
> >  	offset = (void *)&desc->buffer - (void *)desc;
> > -	desc->buffer_size = PAGE_SIZE - offset;
> > +	/*
> > +	 * Some controllers, like JMicron ones, always issue 0x20-byte DMA reads
> > +	 * for descriptors, even 0x10-byte ones. This can cause page faults when
> > +	 * an IOMMU is in use and the oversized read crosses a page boundary.
> > +	 * Work around this by always leaving at least 0x10 bytes of padding.
> > +	 */
> > +	desc->buffer_size = PAGE_SIZE - offset - 0x10;
> >  	desc->buffer_bus = bus_addr + offset;
> >  	desc->used = 0;
> >  
-- 
Stefan Richter
-======----= =-== -==--
http://arcgraph.de/sr/