[PATCH v2 0/2] PCI/DOE: Speed up mailbox polling

[Michal Clapinski <mclapinski@google.com>]

The DOE code waits for mailbox responses by sleeping
PCI_DOE_POLL_INTERVAL (7 ms at HZ=1000, 4 ms at HZ=250) between status
reads. Every DOE-capable endpoint goes through several such exchanges
during enumeration: an abort when the mailbox is created and one
discovery exchange per supported feature. Each pays at least one
interval, even though devices typically respond within about a hundred
microseconds.

Patch 1 fixes PCI_DOE_POLL_INTERVAL being 0 when HZ < 128, which makes
the polling loops spin for up to a second instead of sleeping.

Patch 2 replaces the fixed sleep with an exponential backoff that
busy-waits briefly while high resolution timers are not yet enabled, as
is the case during enumeration at boot. Time spent in pci_device_add()
for an NVIDIA B200 GPU drops from 45.9 ms to 6.4 ms.

Changes in v2:
- New patch 1, fixing busy-polling when HZ < 128.
- Patch 2: share the fast poll with pci_doe_abort() by moving it into
  pci_doe_wait().
- Patch 2: replace the fixed ~2 ms fast-polling window with an
  exponential backoff that busy-waits for its first ~255 us, but only
  until high resolution timers are enabled.

Michal Clapinski (2):
  PCI/DOE: Don't busy-poll the mailbox when HZ < 128
  PCI/DOE: Poll briefly for a response before sleeping

 drivers/pci/doe.c | 49 ++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 44 insertions(+), 5 deletions(-)

-- 
2.56.0.385.gac671c819a-goog