We don't currently have any kind of real acceleration on Ampere GPUs,
but the TTM memcpy() fallback paths aren't really designed to handle
copies between different devices, such as on Optimus systems, and
result in a kernel OOPS.

A few options were investigated to try and fix this, but didn't work
out, and likely would have resulted in a very unpleasant experience
for users anyway.

This commit adds just enough support for setting up a single channel
connected to a copy engine, which the kernel can use to accelerate
the buffer copies between devices.  Userspace has no access to this
incomplete channel support, but it's suitable for TTM's needs.

A more complete implementation of host(fifo) for Ampere GPUs is in
the works, but the required changes are far too invasive that they
would be unsuitable to backport to fix this issue on current kernels.

v2: fix GPFIFO length in RAMFC (reported by Karol)
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: <stable@vger.kernel.org> # v5.12+
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Tested-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210916220406.666454-1-skeggsb@gmail.comSigned-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>