staging/rdma/hfi1: Add support for enabling/disabling PCIe ASPM

staging/rdma/hfi1: Add support for enabling/disabling PCIe ASPM

hfi1 HW has a high PCIe ASPM L1 exit latency and also advertises an
acceptable latency less than actual ASPM latencies. Additional
mechanisms than those provided by BIOS/OS are therefore required to
enable/disable ASPM for hfi1 to provide acceptable power/performance
trade offs. This patch adds this support.

By means of a module parameter ASPM can be either (a) always enabled
(power save mode) (b) always disabled (performance mode) (c)
enabled/disabled dynamically. The dynamic mode implements two
heuristics to alleviate possible problems with high ASPM L1 exit
latency. ASPM is normally enabled but is disabled if (a) there are any
active user space PSM contexts, or (b) for verbs, ASPM is disabled as
interrupt activity for a context starts to increase.

A few more points about the verbs implementation. In order to reduce
lock/cache contention between multiple verbs contexts, some processing
is done at the context layer before contending for device layer
locks. ASPM is disabled when two interrupts for a context happen
within 1 millisec. A timer is scheduled which will re-enable ASPM
after 1 second should the interrupt activity cease. Normally, every
interrupt, or interrupt-pair should push the timer out
further. However, since this might increase the processing load per
interrupt, pushing the timer out is postponed for half a second. If
after half a second we get two interrupts within 1 millisec the timer
is pushed out by another second.

Finally, the kernel ASPM API is not used in this patch. This is
because this patch does several non-standard things as SW workarounds
for HW issues. As mentioned above, it enables ASPM even when advertised
actual latencies are greater than acceptable latencies. Also, whereas
the kernel API only allows drivers to disable ASPM from driver probe,
this patch enables/disables ASPM directly from interrupt context. Due
to these reasons the kernel ASPM API was not used.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Dean Luick <dean.luick@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>