RFC Improving SPI driver latency (vs v3.8.13.14-rt31)

I recently became interested in realtime access to an SPI device on the Odroid U3 platform, with a goal of running a repetitive task every 1ms that performs two SPI transactions totalling around 200 bytes. (for
http://linuxcnc.org/ interfacing to a http://mesanet.com/ "Anything I/O" card)

Unfortunately, I found that there were frequent large latencies, some over 10ms, when using /dev/spidev. This seems to be typical of others' experience using it (for instance, one can find threads discussing disappointing RT performance of SPI on the beaglebone and pandaboard; at least one Raspberry PI project chose to implement a pure userspace SPI driver instead of using spidev)

At all levels of the SPI stack, I found things that could be improved if lowest delays are the goal. I doubt that in their current form these changes are suitable to be incorporated in linux, but I hope that this might spur some discussion that would ultimately lead to better realtime performance of SPI particularly in the preempt-rt kernel.

As you may know, the kernel's spi driver stack consists of
spidev - implementation of the /dev/spidev* device
spi - hardware-independent kernel layer
spi-xyzzy - hardware-dependent driver for xyzzy hardware
(s3c64xx in my device)

I fixed causes of latency at each layer
* First, I eliminated per-ioctl memory allocations in spidev
* Second, I made __spi_sync *actually* synchronous, rather than being a wrapper over spi_async + wait_for_completion; and changed spidev to use spi_sync. This is probably the most controversial aspect of my patches.
* Third, in the hardware-dependent code I moved DMA acquisition to device initialization time rather than transaction time

With these changes, I have been able to run at a 500us repetitive rate, performing two transactions per repetition, with no transaction over 250us, for over 24 hours. This is in contrast to the original performance, in which transactions taking over 10ms were seen multiple times per hour. (24 hours is about 340 million spi transactions) This means there's plenty of margin for the other activities LinuxCNC needs to perform while still running at a 1ms repetitive rate.

I know that 3.8 is by no means current, but 3.8.y is the default kernel shipped by hardkernel for their U3 devices so it was a rational version choice for me. I did skim spi changes from version 3.8 to the present and didn't see anything that looked like it was directed at improving SPI latency, though the underlying code has probably changed enough over time that I assume my patches wouldn't actually apply at the tip of the latest and greatest branches.

Jeff Epler (4):
spi: reenable sync SPI transfers
spidev: Avoid runtime memory allocations
spidev: actually use synchronous transfers
spidev-s3c64xx: allocate dma channel at startup

 drivers/spi/spi-s3c64xx.c | 15 +++++++--------
 drivers/spi/spi.c         | 22 ++++++++++++++++------
 drivers/spi/spidev.c      | 43 ++++++++++++++++++++++++++++++-------------
 3 files changed, 53 insertions(+), 27 deletions(-)

2.1.0

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