[Nouveau] Second copy engine on GF116

[Ilia Mirkin]

Hello,

There's a long-standing bug on nouveau (this is a sample bug, but the
issue has been around for a while:
https://bugs.freedesktop.org/show_bug.cgi?id=85465) whereby we attempt
to use the second PCOPY engine on GF116, and it is sometimes does
nothing, despite mmio register 22500 saying that it's not disabled
(0x22500 == 0 for this user). In the bug you can see a dump from
22400..22600, and all values after 22440 are read as 0. The issue
appears to be more common on mobile GF116's, but I don't know that the
correlation is 100%. No errors are reported by the FIFO or invalid
mmio reads, but the data transfer just does not happen. Switching to
using the first copy engine resolves things, so it's unlikely to be a
more systemic issue in nouveau's usage of the copy engine.

To be clear, when I'm talking about the second PCOPY engine, I'm
talking about the engine at mmio 0x105000, and whose fifo class id is
0x90b8.

Any information on properly detecting that the engine is, in fact,
missing, would be greatly appreciated. Or, conversely, an assurance
that the engine _is_ there on all GF116's and we're just not
initializing something properly, along with perhaps some suggestions
as to what we might be missing.

Thanks,

Ilia Mirkin
imirkin at alum.mit.edu