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

[Lukas Wunner <lukas@xxxxxxxxx>]

Dear Mark,

On Sat, Aug 03, 2019 at 12:10:00PM +0200, Lukas Wunner wrote:
> 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.

Just a gentle ping, this patch set was posted to the list 5 weeks ago,
has all necessary acks and has been tested successfully by 2 people
besides myself.

Do you have any thoughts on it?  Any objections?

In case the patches no longer apply cleanly I've prepared this branch
based on your for-next branch of Aug 23 from which you can merge if
you prefer that:

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

However I can also repost if necessary.

(PS: Apologies for misspelling your name as "Marc" in my e-mail of Aug 24.)

Thanks,

Lukas

> 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