Re: [PATCH 00/10] Raspberry Pi SPI speedups

[Lukas Wunner <lukas@xxxxxxxxx>]

Dear Marc,

to alleviate you of having to add all the Acked-by and Tested-by tags
to this series, I've prepared a "prêt-à-porter" branch complete with
all tags which you can cherry-pick or merge from.

If you decide to instead apply the patches yourself, you can double-check
the result by comparing it to my branch with "git range-diff".

If you have any comments on the series or would like to have anything
changed, please let me know.

Thanks!

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The following changes since commit c55be305915974db160ce6472722ff74f45b8d4e:

  spi: spi-fsl-dspi: Use poll mode in case the platform IRQ is missing (2019-08-23 12:01:44 +0100)

are available in the git repository at:

  https://github.com/l1k/linux bcm2835_spi_simplex_v1

for you to fetch changes up to 37ad33d4bee27d9a24f1deffd675e327d1bb899e:

  spi: bcm2835: Speed up RX-only DMA transfers by zero-filling TX FIFO (2019-08-24 11:54:11 +0200)

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So far the BCM2835 SPI driver cannot cope with TX-only and RX-only
transfers (rx_buf or tx_buf is NULL) when using DMA:  It relies on
the SPI core to convert them to full-duplex transfers by allocating
and DMA-mapping a dummy rx_buf or tx_buf.  This costs performance.

Resolve by pre-allocating reusable DMA descriptors which cyclically
clear the RX FIFO (for TX-only transfers) or zero-fill the TX FIFO
(for RX-only transfers).  Commit fecf4ba3f248 provides some numbers
for the achieved latency improvement and CPU time reduction with an
SPI Ethernet controller.  SPI displays should see a similar speedup.
I've also made an effort to reduce peripheral and memory bus accesses.
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Lukas Wunner (10):
      dmaengine: bcm2835: Allow reusable descriptors
      dmaengine: bcm2835: Allow cyclic transactions without interrupt
      spi: Guarantee cacheline alignment of driver-private data
      spi: bcm2835: Drop dma_pending flag
      spi: bcm2835: Work around DONE bit erratum
      spi: bcm2835: Cache CS register value for ->prepare_message()
      spi: bcm2835: Speed up TX-only DMA transfers by clearing RX FIFO
      dmaengine: bcm2835: Document struct bcm2835_dmadev
      dmaengine: bcm2835: Avoid accessing memory when copying zeroes
      spi: bcm2835: Speed up RX-only DMA transfers by zero-filling TX FIFO

 drivers/dma/bcm2835-dma.c |  38 ++++-
 drivers/spi/spi-bcm2835.c | 408 ++++++++++++++++++++++++++++++++++++++--------
 drivers/spi/spi.c         |  18 +-
 3 files changed, 390 insertions(+), 74 deletions(-)