mirror of
git://projects.qi-hardware.com/ben-wpan.git
synced 2024-11-22 18:41:32 +02:00
176 lines
6.1 KiB
Plaintext
176 lines
6.1 KiB
Plaintext
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.
|
|
|
|
Note that the code referenced in this analysis is meant to illustrate
|
|
the general concepts. Some variants have been dropped since and the
|
|
ones still in existence have been cleaned up and debugged in
|
|
preparation for upstream submission. The current versions can be
|
|
found in https://github.com/wpwrak/ben-wpan-linux
|
|
|
|
|
|
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
|