Am 03.08.19 um 12:10 schrieb Lukas Wunner:
> 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
>
Acked-by: Stefan Wahren <wahrenst@xxxxxxx>
Sorry, for this late reply
