[ Also posted on the qi-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:
AbbreviationFramework Transport Chip driver
--- --- --- ---
spi-gpiospi_atben_gpio spi-gpioat86rf230
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_atben10 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 25492559
spi-jz4740-gpio 36764308
spi_atben 36764327
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_atben423
Computational cost
--
The high-level operations of sending and receiving produce the
following major low-level operations:
Operation registerbuffer waitqueue
readwrite readwrite
--- --- --- --- --- -
reception 1 - 1 - 1
transmission9 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 26717771866116626652727
spi-gpio-atben 186 561 583 86812401256
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/receive
