[HowTo] Enable full duplex SPI

[Cosik]

Hi,

In my project I need to use full duplex communication with RF module, so I started looking how to use SPI.

I didn't test it on image distributed by Olimex, because I need to use custom kernel.

At first we need to download kernel source, for example:

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git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

Next we should perform initial kernel configuration for A20:

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make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- sun7i_defconfig

Now we need to patch our kernel in few steps:
1. create file linux-sunxi/drivers/spi/spi-sun7i.c with this content
spi-sun7i.c

2. To linux-sunxi/drivers/spi/Makefile at the end add:

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obj-$(CONFIG_SPI_SUN7I)         += spi-sun7i.o

3. Update linux-sunxi/drivers/spi/Kconfig:

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config SPI_SUN7I
   tristate "SUN7I SPI Controller"
   depends on ARCH_SUN7I
   help
      Allwinner Soc SPI controller,present on SUN7I chips.

config SUN7I_SPI_NDMA
        bool "SUN7I SPI Normal DMA mode select"
        depends on SPI_SUN7I
        help
          This selects SPI DMA mode with DMA transfer
          Y select NDMA mode and N select DDMA mode

4. Update kernel config

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CONFIG_SPI_SUN7I=y
CONFIG_SUN7I_SPI_NDMA=y
It's important to have hard compiled spidev in kernel.

Now we can compile kernel with command:

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make -j4 ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- uImage modules
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- INSTALL_MOD_PATH=output modules_install

Also we need to update FEX file, important is to set full_duplex to 1, example:

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[spi_board3]
modalias = "spidev"
max_speed_hz = 12000000
bus_num = 3
chip_select = 0
mode = 0
full_duplex = 1
manual_cs = 0

Now we can test install kernel, modules and then test them. For test I used pyA20 and kernel/linux-sunxi/Documentation/spi/spidev_fdx.c

To get working correct spi, we also need to update /usr/include/linux/spi/spidev.h on our board with:

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struct spi_ioc_transfer {
__u64 tx_buf;
__u64 rx_buf;

__u32 len;
__u32 speed_hz;

__u16 interbyte_usecs;
__u16 delay_usecs;
__u8 bits_per_word;
__u8 cs_change;
__u32 pad;
}

Test results:
pyA20 from Olimex didn't correct support full duplex.

Here are commend which I used to test SPI in python

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>>> from pyA20 import spi
>>> spi.open("/dev/spidev1.0")
>>> spi.xfer([0x01, 0x02], 2)
[14, 8]

Here are results

index	start-time	end-time	event?		MOSI data	MISO data
0	20,00 ns	20,00 ns	true	CS_LOW
1	14,60 μs	14,60 μs	false		0
2	14,60 μs	14,60 μs	false			14
3	35,94 μs	35,94 μs	false		0
4	35,94 μs	35,94 μs	false			8
5	115,92 μs	115,92 μs	false		1
6	115,92 μs	115,92 μs	false			8
7	137,26 μs	137,26 μs	false		2
8	137,26 μs	137,26 μs	false			8
9	418,80 μs	418,80 μs	true	CS_HIGH

Problem is that this function first read SPI and then send data, and didn't return read data during sending data.

spidev_fdx is working better but not enough good for me, here are results.
Command:

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./spidev_fdx -v -m1 -r2 /dev/spidev1.0
/dev/spidev1.0: spi mode 0, 8 bits per word, 100000 Hz max
response( 1,  2):  08
read( 2,  2): 0e 08,

index	start-time	end-time	event?		MOSI data	MISO data
0	20,00 ns	20,00 ns	true	CS_LOW
1	13,98 μs	13,98 μs	false		0
2	13,98 μs	13,98 μs	false			14
3	155,16 μs	155,16 μs	false		0
4	155,16 μs	155,16 μs	false			8
5	259,48 μs	259,48 μs	true	CS_HIGH
6	430,06 μs	430,06 μs	true	CS_LOW
7	443,22 μs	443,22 μs	false		0
8	443,22 μs	443,22 μs	false			14
9	464,54 μs	464,54 μs	false		0
10	464,54 μs	464,54 μs	false			8
11	513,76 μs	513,76 μs	true	CS_HIGH

References:
Kernel compile
Fex edit
Orginal information source

Feedback is expected

[Mouchon]

Did you test by looping MISO to MOSI ?

[Cosik]

Hi,

No, I tested it with real device. 0x0E is device status and 0x08 is default value in 0x00 register.

Or you suggest to perform test on loop back?

[Mouchon]

Or you suggest to perform test on loop back?

Can be interesting too but that was only a question on how you perform your test :-)

[Mouchon]

I did some test with the following test c code

https://drive.google.com/file/d/0BwSBxMZkVQwMVXBlRjlDWHVEbW8/view?usp=sharing

I put a wire between MOSI and MISO. here is the result.

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./spi-test -C -L -D /dev/spidev2.0  0x10,0x20,0x30
10 20 30
10 20 30


So seem to work.

I have also try with python code but i have an issue

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Python 2.7.3 (default, Mar 14 2014, 17:55:54)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from pyA20 import spi
>>> spi.open("/dev/spidev2.0")
>>> spi.xfer([0x01, 0x02], 2)

Segmentation fault



So have an issue with the python code.
I run the test on a 3.4.103+ kernel compiled with information you provide.

[Cosik]

Hi,

I also tested it on 3.4.103 and everything is "working". Did you compile pyA20 on new kernel and update spidev.h? If you could, pleas show me yours spidev.h