Re: Possible issue with dangling PCI BARs

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2007-12-13 at 12:04 -0800, Jesse Barnes wrote:
> 
> Yeah, that seems like a reasonable compromise.  Though in practice
> I'd 
> expect the full disable decode approach to work fairly well too.  I 
> mean, if we really end up failing to allocate space for the device
> with 
> the root drive on it, there are probably bigger issues than just 
> failing to get a few bytes of I/O space for it...
> 

The really bad scenario would be something like the Sil680 that Alan
talked about setup by a BIOS that "knows" about the unused BAR when
MMIO_EN is not set.

If the device is behind a P2P bridge and the BIOS has set the windows of
that bridge so tightly that there is no room to allocate the MMIO BAR,
then a full disable/full enable would fail on a device that would
otherwise work using only PIO.

However, I'd be curious to see that happening in practice :-)

But I think it's fair enough to do an IO only / MEM only approach. I've
seen cases where IO is just not useable because of other constraints and
so I expect the MEM-only case to be more common, especially on non-x86.

Ben.