[PATCH v4 0/6] spi: spi-fsl-dspi: Target mode improvements

[James Clark <james.clark@linaro.org>]

Improve usability of target mode by reporting FIFO errors and increasing
the buffer size when DMA is used. While we're touching DMA stuff also
switch to non-coherent memory, although this is unrelated to target
mode.

The first commit is marked as a fix because it can fix intermittent
issues with existing transfers, rather than the later fixes which
improve larger than FIFO target mode transfers which would have never
worked.

With the combination of the commit to increase the DMA buffer size and
the commit to use non-coherent memory, the host mode performance figures
are as follows on S32G3:

  # spidev_test --device /dev/spidev1.0 --bpw 8 --size <test_size> --cpha --iter 10000000 --speed 10000000

  Coherent (4096 byte transfers): 6534 kbps
  Non-coherent:                   7347 kbps

  Coherent (16 byte transfers):    447 kbps
  Non-coherent:                    448 kbps

Just for comparison running the same test in XSPI mode:

  4096 byte transfers:            2143 kbps
  16 byte transfers:               637 kbps

These tests required hacking S32G3 to use DMA in host mode, although
the figures should be representative of target mode too where DMA is
used. And the other devices that use DMA in host mode should see similar
improvements.

Signed-off-by: James Clark <james.clark@linaro.org>
---
Changes in v4:
- Fix !CONFIG_DMA_ENGINE build (and actually test it this time)
- Reword completion counter comment
- Reword some commit messages
- Reset tries in dspi_poll() for each transfer
- Check for fifo errors in dspi_poll() before checking for completion
- Link to v3: https://lore.kernel.org/r/20250624-james-nxp-spi-dma-v3-0-e7d574f5f62c@linaro.org

Changes in v3:
- Stub out DMA functions in the driver so no-DMA builds work
- Link to v2: https://lore.kernel.org/r/20250613-james-nxp-spi-dma-v2-0-017eecf24aab@linaro.org

Changes in v2:
- Store status in cur_msg->status rather than adding xfer_status
- Show exact underflow/overflow flags in error message
- Rate limit error messages
- Add a comment about resetting the completion counter prior to transfer
- Rename dspi_is_fifo_overflow() -> dspi_fifo_error()
- Add performance figures to cover letter
- Rebase onto https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git/for-next
  to avoid some conflicts
- Link to v1: https://lore.kernel.org/r/20250609-james-nxp-spi-dma-v1-0-2b831e714be2@linaro.org

---
James Clark (5):
      spi: spi-fsl-dspi: Clear completion counter before initiating transfer
      spi: spi-fsl-dspi: Store status directly in cur_msg->status
      spi: spi-fsl-dspi: Stub out DMA functions
      spi: spi-fsl-dspi: Use non-coherent memory for DMA
      spi: spi-fsl-dspi: Report FIFO overflows as errors

Larisa Grigore (1):
      spi: spi-fsl-dspi: Increase DMA buffer size

 drivers/spi/spi-fsl-dspi.c | 233 ++++++++++++++++++++++++++++++++-------------
 1 file changed, 166 insertions(+), 67 deletions(-)
---
base-commit: 4f326fa6236787ca516ea6eab8e5e9dc5c236f03
change-id: 20250522-james-nxp-spi-dma-a997ebebfb6b

Best regards,
-- 
James Clark <james.clark@linaro.org>