Performance analysis of SPI drivers for ATBEN
Werner Almesberger <
werner@...>
2013-04-10 14:43:23 GMT
[ Also posted on the linux-zigbee hardware list. No cross-post, since
few people will be on both lists. ]
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/master/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/master/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/master/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/master/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
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