diff --git a/atben/misc/atben-spi-performance.txt b/atben/misc/atben-spi-performance.txt new file mode 100644 index 0000000..a8dfd8b --- /dev/null +++ b/atben/misc/atben-spi-performance.txt @@ -0,0 +1,173 @@ +Performance analysis of SPI drivers for ATBEN +============================================= + +This was originally posted on April 10 2013 to the linux-zigbee and +the qi-hardware list. The URLs have been updated for stability. + +What this is all about +---------------------- + +The ATBEN board on the Ben NanoNote needs a bit-banging SPI driver. +There are several ways to implement this, ranging from reuse of the +generic spi-gpio driver to an optimized driver that's specific for +this platform. + +I implemented several such approaches and measured their performance +in the Ben NanoNote. Below are my findings. + +Comments welcome. + + +Cast and characters +------------------- + +spi_atben_gpio: NanoNote-specific framework for setting up the +AT86RF230/1 with SPI-GPIO or one of the optimized drivers (below). +The name derives from spi_atben (see below) and should be changed +(maybe to atben_spi or atben_spi_gpio ?) since it is not an SPI +driver but merely a framework that provides configuration data and +performs miscellaneous platform setup. +https://github.com/wpwrak/ben-wpan-linux/blob/perfcomp/drivers/net/ieee802154/spi_atben_gpio.c + +spi_atben: like spi_atben_gpio, but contains a highly optimized +SPI driver for the ATBEN configuration in the Ben NanoNote. +https://github.com/wpwrak/ben-wpan-linux/blob/perfcomp/drivers/net/ieee802154/spi_atben.c + +spi-jz4740-gpio: SPI-GPIO driver optimized for the Jz4740. Uses the +optimized register accesses from spi_atben but pin assignment is not +restricted to ATBEN. The only limitation is that MOSI, MISO, and +SCLK must be on the same port. +https://github.com/wpwrak/ben-wpan-linux/blob/perfcomp/drivers/spi/spi-jz4740-gpio.c + +spi-gpio-atben: task-specific SPI-GPIO driver using the #include +"spi-gpio.c" method. Replaces gpiolib functions with register +accesses specific to the ATBEN configuration in the Ben NanoNote. +Note that some of the code could be moved into Jz4740 +architecture-specific GPIO support. +https://github.com/wpwrak/ben-wpan-linux/blob/perfcomp/drivers/spi/spi-gpio-atben.c + +In the following sections, we abbreviate the stack configurations +as follows: + +Abbreviation Framework Transport Chip driver +--------------- --------------- --------------- ----------- +spi-gpio spi_atben_gpio spi-gpio at86rf230 +spi-gpio-atben spi_atben_gpio spi-gpio-atben at86rf230 +spi-jz4740-gpio spi_atben_gpio spi-jz4740-gpio at86rf230 +spi_atben spi_atben at86rf230 + + +Measurements +------------ + +Access time to AT86RF231 registers and buffer, in microseconds, on +an otherwise idle Ben NanoNote: + +Driver read from 0x51 read 120 bytes from buffer +| | write 0x0a to 0x15 write 1 byte to buffer (0x33) +| | | read 1 byte from buffer write 120 bytes +| | | | | | | +spi-gpio 81 85 186 1696 97 1596 +spi-gpio-atben 63 59 123 498 65 437 +spi-jz4740-gpio 10 8 21 280 10 231 +spi_atben 10 7 21 280 10 230 + +Data rate for hypothetical buffer accesses of infinite length. +I.e., kbps = 1000*119*8/(t_write120-t_write1) + +Driver buffer read (kbps) buffer write (kbps) +--------------- ----------------------- ------------------- +spi-gpio 630 635 +spi-gpio-atben 2549 2559 +spi-jz4740-gpio 3676 4308 +spi_atben 3676 4327 + +At the air interface, IEEE 802.15.4 has a data rate of 250 kbps. +The AT86RF231 transceiver also supports non-standard higher data +rates up to 2 Mbps. + +Driver(s) Code size (lines) +--------------------------------------- ----------------- +spi_atben_gpio 128 +spi_atben_gpio + spi-gpio-atben 128+ 53 +spi_atben_gpio + spi-jz4740-gpio 128+416 +spi_atben 423 + + +Computational cost +------------------ + +The high-level operations of sending and receiving produce the +following major low-level operations: + +Operation register buffer waitqueue + read write read write +--------------- ------- ------- ------- ------- --------- +reception 1 - 1 - 1 +transmission 9 4 - 1 1 + +Using the measured data from above, we get the following total +computational overhead in microseconds, without considering the +waitqueue scheduling delay: + +Driver reception transmission + 1 120 127 1 120 125 (bytes) +--------------- ------- ------- ------- ------- ------- ------- +spi-gpio 267 1777 1866 1166 2665 2727 +spi-gpio-atben 186 561 583 868 1240 1256 +spi-jz4740-gpio 31 290 304 132 353 362 + +Note that the minimum frame length in IEEE 802.15.4 is 5 bytes. +The values for 125 (excluding CRC) and 127 (including CRC) bytes +are extrapolated. + +According to [1], maximum-sized frames can be sent/received, +including CSMA/CA and acknowledgement, at a rate between one +every 4928 us and one every 7168 us. + +We would therefore get the following maximum CPU load: + +Driver Reception Transmission +--------------- --------------- ------------ +spi-gpio 38% 55% +spi-gpio-atben 12% 25% +spi-jz4740-gpio 6% 7% + + +Observations +------------ + +spi-gpio needs the smallest amount of new code but is also very +inefficient, making it questionable whether this configuration +would yield acceptable performance in regular use. + +With spi-gpio-atben, only a small amount of code is added, but +buffer accesses become almost 4 times faster. Register reads and +writes are still fairly slow. + +spi_atben and spi-jz4740-gpio both achieve the best performance +without significant differences between them. Both add a complete +SPI driver. Of the two, spi-jz4740-gpio is preferable, because it +uses the nearly universal spi_atben_gpio framework driver. + + +Conclusion +---------- + +I think performance trumps most other considerations in this case. +spi-gpio is clearly too inefficient. spi_atben_gpio with +spi-jz4740-gpio offers the best performance and has a low impact +on the system load (< 10%). In case this solution would be met +with strong resistance for some reason, spi-gpio-atben would offer +a compromise between performance and the amount of code. + + +References +---------- + +[1] http://www.jennic.com/files/support_files/JN-AN-1035%20Calculating%20802-15-4%20Data%20Rates-1v0.pdf + +_______________________________________________ +Qi Hardware Discussion List +Mail to list (members only): discussion@lists.en.qi-hardware.com +Subscribe or Unsubscribe: http://lists.en.qi-hardware.com/mailman/listinfo/discussion