[PATCH 00/10] Raspberry Pi SPI speedups

[Lukas Wunner <lukas@xxxxxxxxx>]

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).  Patch [07/10] 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.

The series is meant to be applied on top of broonie/for-next.
It can be applied to Linus' current tree if commit
8d8bef503658 ("spi: bcm2835: Fix 3-wire mode if DMA is enabled")
is cherry-picked from broonie's repo beforehand.

Please review and test.  Thank you.

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(-)

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2.20.1