[PATCH net-next 0/2] net: mana: Avoid DMA queue allocation failure under memory fragmentation

The MANA driver can fail to bring up its queues on systems with high
memory utilization because every GDMA queue ring is allocated as a
single dma_alloc_coherent() of the whole power-of-2 ring size. Under
memory fragmentation these high-order allocations may fail, preventing
the driver from creating queues when opening the interface, after a VF
reset, or when reconfiguring channels, ring parameters or MTU.

Per-queue sizes that are problematic, with depth and size given as
(default, max) over the ethtool ring settings:

  ring                  entry  depth          size
  ------------------------------------------------------------
  TX completion queue   64 B   (256, 16384)   (16 KB, 1024 KB)
  TX send queue         32 B   (256, 16384)   ( 8 KB,  512 KB)
  RX completion queue   64 B   (1024, 8192)   (64 KB,  512 KB)
  RX receive queue      32 B   (1024, 8192)   (32 KB,  256 KB)
  event queue           16 B   2048 (fixed)   32 KB

This series addresses the issue by:
  1. Routing all CPU-side ring access through mana_gd_ring_ptr() and
     mana_gd_ring_contig_avail(). On a contiguous ring these reduce to
     simple arithmetic, so this patch is a pure refactor.
  2. Falling back in mana_gd_alloc_memory() to a vector of scattered
     order-0 coherent pages when the contiguous allocation fails. The
     device sees the same page-list format either way, as
     mana_gd_create_dma_region() already describes a ring as a list of
     MANA_PAGE_SIZE addresses. The HW channel stays contiguous, as
     advertising a scattered page list needs the HW channel itself.

Throughput testing confirms no regression. Since the fallback only
triggers under memory fragmentation, the scattered-page path was enabled
unconditionally for all eligible GDMA queue rings during testing (iperf3,
Gbit/s):

                 Baseline    Patched     Patched
  Connections   Contiguous  Contiguous  Scattered
  -----------------------------------------------
  1                  46.1        46.2       46.1
  16                 182         182        182
  32                 182         182        182
  64                 182         182        182

Aditya Garg (2):
  net: mana: Route ring-buffer access through offset-based helpers
  net: mana: Fall back to scattered pages for GDMA queues

 .../net/ethernet/microsoft/mana/gdma_main.c   | 219 +++++++++++++++---
 .../net/ethernet/microsoft/mana/hw_channel.c  |   2 +-
 drivers/net/ethernet/microsoft/mana/mana_en.c |   3 +
 include/net/mana/gdma.h                       |  19 +-
 4 files changed, 205 insertions(+), 38 deletions(-)

-- 
2.43.0