Hi Jeff, thanks for your effort of investigating this issue. I'll have a look if it can help me decreasing SPI latency with my ARM based boards (Gumstix, BeagleBoneBlack). Have you looked into setting a RTPRIO for the SPI worker threads (spi_init_queue() in spi.c)? As far as I see hardly any SPI hardware-dependent driver sets the "rt" property in the respective spi_master struct, so that the main SPI driver (spi.c) will not use a RT priority, which will be MAX_RT_PRIO - 1. So far setting a medium RTPRIO for the SPI worker threads helped a lot to reduce latency. Now I'm able to sustain a stable SPI transaction rate of around 800 Hz. Cheers Phil -------- Original Message -------- Subject: [RFC 0/4] Improving SPI driver latency (vs v3.8.13.14-rt31) From: Jeff Epler <jepler@xxxxxxxxxxxxxx> To: linux-rt-users@xxxxxxxxxxxxxxx Date: 09/01/2014 04:30 PM > 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. (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 preempt-rt, but I hope that > this might spur some discussion that would ultimately lead to better > realtime performance of SPI 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 > * Third, in the hardware-dependent code I moved DMA acquisition to > device initialization time rather than transaction time > > I did not quite achieve my goal of a 1ms repetitive rate yet, but with > these changes I have run for 12+ hours at a rate of 3 transactions per > 2ms with acceptable worst-case performance---under 250us for the biggest > transaction, and 465us for all three (they have different sizes), with > typical figures of more like 200us for all three transactions. This is > in contrast to the original performance, in which transactions taking > over 10ms were seen multiple times per hour. (12 hours is about 64 > million spi transations) > > (I changed from talking about 2 transactions to 3, because for an > unrelated reason the communication in my program is currently divided > into 3 SPI transactions when two would do) > > 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. > > PS The fact that the first PREEMPT-RT kernel I built for the odroid > worked and has basically good latency (until trying to talk to the > hardware :-/) impressed the heck out of me. > > 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(-) > -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
