spi_bcm2835 & 3wire mode support

[fsr]

Hi,

I've encountered and error which I am unable to resolve and was wondering if anyone can shed some light on it.

According to the kernel sources the spi_bcm2835 module supports wire mode https://github.com/raspberrypi/linux/bl ... -bcm2835.c

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#define BCM2835_SPI_MODE_BITS	(SPI_CPOL | SPI_CPHA | SPI_CS_HIGH \
				| SPI_NO_CS | SPI_3WIRE)

I can set the flag without error but an actual transfer fails with errno 0x16 'Invalid Parameter'.

Here's my current config - have I missed something obvious?

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fraser@rpi:~/src/ref $ uname -a
Linux rpi 4.1.13-v7+ #826 SMP PREEMPT Fri Nov 13 20:19:03 GMT 2015 armv7l GNU/Linux

fraser@rpi:~/src/ref $ grep spi /boot/config.txt
dtparam=spi=on

fraser@rpi:~/src/ref $ lsmod | grep spi
spi_bcm2835             7216  0 

fraser@rpi:~/src/ref $ grep spi /etc/group
spi:x:999:pi,fraser

fraser@rpi:~/src/ref $ ls -l /dev/spi*
crw-rw---- 1 root spi 153, 0 Jan 13 13:11 /dev/spidev0.0
crw-rw---- 1 root spi 153, 1 Jan 13 13:11 /dev/spidev0.1

fraser@rpi:~/src/ref $ ./spidev_test -D /dev/spidev0.0 
spi mode: 0
bits per word: 8
max speed: 500000 Hz (500 KHz)

00 00 00 00 00 00 
00 00 00 00 00 00 
00 00 00 00 00 00 
00 00 00 00 00 00 
00 00 00 00 00 00 
00 00 00 00 00 00 
00 00 

fraser@rpi:~/src/ref $ ./spidev_test -D /dev/spidev0.0 -3
spi mode: 16
bits per word: 8
max speed: 500000 Hz (500 KHz)
can't send spi message: Invalid argument
Aborted

[joan]

SPI_CS_HIGH and SPI_NO_CS sound like incompatible flags. Are you sure it's complaining about 3-wire mode?

I see nothing specifying the number of bytes to be sent before switching to 3-wire to receive. How does the interface work?

[fsr]

SPI_CS_HIGH and SPI_NO_CS sound like incompatible flags.

They are modes the driver can support. The BCM2835_SPI_MODE_BITS just a list of capabilities for spidev (which actually creates the /dev node). see line 783 of the spi-bcm2835.c when this list is given to spidev as part of the initialiation.

I'm only setting 3wire with the -3 as per:
./spidev_test -D /dev/spidev0.0 -3
you'll se the output of that command shows the mode is = 16 ( = 0x10 = just SPI_3WIRE )
(FYI the spidev_test program is part of the kernel documentation https://www.kernel.org/doc/Documentatio ... dev_test.c I'm just using it as it rules out anything bodgy I may have done :)

I see nothing specifying the number of bytes to be sent before switching to 3-wire to receive. How does the interface work?

ah ok, thats a thought.
by default the spi messages are full duplex. I thought (assumed!) with 3wire mode enabled the driver would be smart enough to do the sending and then do the receiving - perhaps not. And yes good point, how does it know how many bytes to transfer in each direction? a message only has one length. The spidev.h does vaguely mention half duplex so I'll explore that a little and see where it takes me.

cheers,
F.

[joan]

For reference I drive the device directly and implement 3-wire mode. See http://abyz.co.uk/rpi/pigpio/cif.html#spiOpen

[fsr]

FYI I think I have sussed it out now.
I still need to do some testing but I can at least send and receive messages without triggering errors now.

For future reference:
The messages need to be constructed in a particular way when 3wire mode is active.

This way does not work (I assume as it implies a full duplex transaction which can't be done in 3wire mode):

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ioctl(fd, SPI_IOC_RD_MODE32, 0x10); //enable 3wire mode

struct spi_ioc_transfer tr;  //structure to hold message

tr.tx_buf = (unsigned long)tx; //pointer to send buffer 
tr.rx_buf = (unsigned long)rx; //pointer to receive buffer 
tr.len = len;  //size of the buffers

ioctl(fd, SPI_IOC_MESSAGE(1), &tr); //results in errno0x16 invalid argument

This way is correct as it implies a half duplex transaction

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ioctl(fd, SPI_IOC_RD_MODE32, 0x10); //enable 3wire mode

struct spi_ioc_transfer tr[2];  //structure to hold messages

tr[0].tx_buf = (unsigned long)tx; //pointer to send buffer 
tr[0].rx_buf = 0;
tr[0].len = len;  //size of the transmit buffer

tr[1].tx_buf = 0;
tr[1].rx_buf = (unsigned long)rx; //pointer to receive buffer 
tr[1].len = len;  //size of the receive buffer

ioctl(fd, SPI_IOC_MESSAGE(2), &tr); //no error

Also note it would appear the spidev_test.c sample that is part of the kernel documentation does not work in 3wire mode and so can't be considered a true test.