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

[Aditya Garg <gargaditya@linux.microsoft.com>]

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