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(-)
--
2.20.1
