Re: [net-next v4 00/12] Add TSO map-once DMA helpers and bnxt SW USO support

On Fri, Mar 20, 2026 at 8:11 PM Joe Damato [off-list ref] wrote:

Greetings:

This series extends net/tso to add a data structure and some helpers allowing
drivers to DMA map headers and packet payloads a single time. The helpers can
then be used to reference slices of shared mapping for each segment. This
helps to avoid the cost of repeated DMA mappings, especially on systems which
use an IOMMU. N per-packet DMA maps are replaced with a single map for the
entire GSO skb. As of v3, the series uses the DMA IOVA API (as suggested by
Leon [1]) and provides a fallback path when an IOMMU is not in use. The DMA
IOVA API provides even better efficiency than the v2; see below.

The added helpers are then used in bnxt to add support for software UDP
Segmentation Offloading (SW USO) for older bnxt devices which do not have
support for USO in hardware. Since the helpers are generic, other drivers
can be extended similarly.

The v2 showed a ~4x reduction in DMA mapping calls at the same wire packet
rate on production traffic with a bnxt device. The v3, however, shows a larger
reduction of about ~6x at the same wire packet rate. This is thanks to Leon's
suggestion of using the DMA IOVA API [1].

Special care is taken to make bnxt ethtool operations work correctly: the ring
size cannot be reduced below a minimum threshold while USO is enabled and
growing the ring automatically re-enables USO if it was previously blocked.

I've extended netdevsim to have support for SW USO, but I used
tso_build_hdr/tso_build_data in netdevsim because I couldn't figure out if
there was a way to test the DMA helpers added by this series. If anyone has
suggestions, let me know. I think to test the DMA helpers you probably need
to use real hardware.

Since this v4 made minor updates to the python test (see below and patch 12
changelog), I re-ran the test with both netdevsim and real bnxt hardware and
the test passed.

Thanks,
Joe

Thanks Joe. Its nice work! I reviewed the series and added my Rb for
all patches except core changes (which btw also loog good to me, but I
think other experts may review)

[1]: https://lore.kernel.org/netdev/20260316194419.GH61385@unreal/ (local)

v4:
  - Fixed kdoc issues in patch 2. No functional change.
  - Added Pavan's Reviewed-by to patches 3, 4, and 5.
  - Fixed the issue Pavan (and the AI review) pointed out in patch 8. See
    patch changelog.
  - Added parentheses around gso_type check in patch 11 for clarity. No
    functional change.
  - Fixed python linter issues in patch 12. No functional change.

v3: https://lore.kernel.org/netdev/20260318191325.1819881-1-joe@dama.to/ (local)
  - Converted from RFC to an actual submission.
  - Updated based on Leon's feedback to use the DMA IOVA API. See individual
    patches for update information.

RFCv2: https://lore.kernel.org/netdev/20260312223457.1999489-1-joe@dama.to/ (local)
  - Some bugs were discovered shortly after sending: incorrect handling of the
    shared header space and a bug in the unmap path in the TX completion.
    Sorry about that; I was more careful this time.
  - On that note: this rfc includes a test.

RFCv1: https://lore.kernel.org/netdev/20260310212209.2263939-1-joe@dama.to/ (local)

Joe Damato (12):
  net: tso: Introduce tso_dma_map
  net: tso: Add tso_dma_map helpers
  net: bnxt: Export bnxt_xmit_get_cfa_action
  net: bnxt: Add a helper for tx_bd_ext
  net: bnxt: Use dma_unmap_len for TX completion unmapping
  net: bnxt: Add TX inline buffer infrastructure
  net: bnxt: Add boilerplate GSO code
  net: bnxt: Implement software USO
  net: bnxt: Add SW GSO completion and teardown support
  net: bnxt: Dispatch to SW USO
  net: netdevsim: Add support for SW USO
  selftests: drv-net: Add USO test

 drivers/net/ethernet/broadcom/bnxt/Makefile   |   2 +-
 drivers/net/ethernet/broadcom/bnxt/bnxt.c     | 190 +++++++++---
 drivers/net/ethernet/broadcom/bnxt/bnxt.h     |  33 +++
 .../net/ethernet/broadcom/bnxt/bnxt_ethtool.c |  19 +-
 drivers/net/ethernet/broadcom/bnxt/bnxt_gso.c | 236 +++++++++++++++
 drivers/net/ethernet/broadcom/bnxt/bnxt_gso.h |  31 ++
 drivers/net/netdevsim/netdev.c                | 100 ++++++-
 include/linux/skbuff.h                        |  11 +
 include/net/tso.h                             |  61 ++++
 net/core/tso.c                                | 273 ++++++++++++++++++
 tools/testing/selftests/drivers/net/Makefile  |   1 +
 tools/testing/selftests/drivers/net/uso.py    |  96 ++++++
 12 files changed, 1013 insertions(+), 40 deletions(-)
 create mode 100644 drivers/net/ethernet/broadcom/bnxt/bnxt_gso.c
 create mode 100644 drivers/net/ethernet/broadcom/bnxt/bnxt_gso.h
 create mode 100755 tools/testing/selftests/drivers/net/uso.py


base-commit: 9f4960b94f1a044f76da98a765d6cbd294c22c92
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2.52.0