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

[Pavan Chebbi]

On Fri, Mar 20, 2026 at 8:11 PM Joe Damato <joe@xxxxxxx> 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/
>
> 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@xxxxxxx/
>   - 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@xxxxxxx/
>   - 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@xxxxxxx/
>
> 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
> --
> 2.52.0
>

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