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Re: [Xen-devel] pci adapter not working in dom0. works under plain Linux.

On Tue, Jun 10, 2014 at 11:19:18AM +0000, James Harper wrote:
> > 
> > On Sun, Jun 08, 2014 at 11:01:25AM +0000, James Harper wrote:
> > > I bought an unsupported DVB card (by mistake - I bought the v2 version
> > instead of the supported v1 version), and I've just finished getting it 
> > working
> > under Linux, but spent the last day or so trying to figure out why it keeps
> > dropping buffers even though the reception is perfect. As soon as I noticed
> > that the lost data is always in PAGE_SIZE multiples, I figured I'd better 
> > boot in
> > plain Linux instead of Xen, and when I did that it worked perfectly.
> > 
> > What happens if you boot dom0 with 'dom0_mem=max:1G' and limit the
> > amount of memory
> > Xen sees by using 'mem=4G' on the Xen command line?
> > 
> That seems to make the problem go away. The machine itself has 5GB of memory 
> in it, so (assuming an even spread) only 1/5 of allocations would come from 
> >4GB memory right? This matches the missing pages of data in the resulting 
> DMA transfers.

That was my theory.
> Where should I start looking for the fix? The card is PCIe so I would have 
> thought it would have supported DMA to memory > 4GB, is that the problem? I 
> think one of my USB devices has the same problem too.

Ugh. They all should do the right job - that is using the DMA API.
> Btw, what is dom0_mem=max:1G supposed to do? Dom0 still has ~3.7GB memory 
> allocated to it on boot.

Oh, I think I mentioned that it should have been on the 'dom0' line but
I meant Xen line.

If you try removing the 'mem=4G' and just use 'dom0_mem=max:1G' please?
That should limit the amount of memory the dom0 uses (it sees the
rest of the E820 - but does not use it) - and that means that the
drivers should _also_ use said below memory.

I had some preliminary patches for the V4L API to use a better
DMA ops, but I fear that they are bitrotten (and I can't even
find them!). But lets try first determine if the workarounds
work for you.
> Thanks!
> James

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