SPI linux driver - linux

Question

I am trying to learn how to write a basic SPI driver and below is the probe function that I wrote.
What I am trying to do here is setup the spi device for fram(datasheet) and use the spi_sync_transfer()api description to get the manufacturer's id from the chip.
When I execute this code, I can see the data on the SPI bus using logic analyzer but I am unable to read it using the rx buffer. Am I missing something here? Could someone please help me with this?
static int fram_probe(struct spi_device *spi)
{
int err;
unsigned char ch16[] = {0x9F,0x00,0x00,0x00};// 0x9F => 10011111
unsigned char rx16[] = {0x00,0x00,0x00,0x00};
printk("[FRAM DRIVER] fram_probe called \n");
spi->max_speed_hz = 1000000;
spi->bits_per_word = 8;
spi->mode = (3);
err = spi_setup(spi);
if (err < 0) {
printk("[FRAM DRIVER::fram_probe spi_setup failed!\n");
return err;
}
printk("[FRAM DRIVER] spi_setup ok, cs: %d\n", spi->chip_select);
spi_element[0].tx_buf = ch16;
spi_element[1].rx_buf = rx16;
err = spi_sync_transfer(spi, spi_element, ARRAY_SIZE(spi_element)/2);
printk("rx16=%x %x %x %x\n",rx16[0],rx16[1],rx16[2],rx16[3]);
if (err < 0) {
printk("[FRAM DRIVER]::fram_probe spi_sync_transfer failed!\n");
return err;
}
return 0;
}

Answer 1

spi_element is not declared in this example. You should show that and also how all elements of that are array are filled. But just from the code that's there I see a couple mistakes.
You need to set the len parameter of spi_transfer. You've assigned the TX or RX buffer to ch16 or rx16 but not set the length of the buffer in either case.
You should zero out all the fields not used in the spi_transfer.
If you set the length to four, you would not be sending the proper command according to the datasheet. RDID expects a one byte command after which will follow four bytes of output data. You are writing a four byte command in your first transfer and then reading four bytes of data. The tx_buf in the first transfer should just be one byte.
And finally the number of transfers specified as the last argument to spi_sync_transfer() is incorrect. It should be 2 in this case because you have defined two, spi_element[0] and spi_element[1]. You could use ARRAY_SIZE() if spi_element was declared for the purpose of this message and you want to sent all transfers in the array.
Consider this as a way to better fill in the spi_transfers. It will take care of zeroing out fields that are not used, defines the transfers in a easy to see way, and changing the buffer sizes or the number of transfers is automatically accounted for in remaining code.
const char ch16[] = { 0x8f };
char rx16[4];
struct spi_transfer rdid[] = {
{ .tx_buf = ch16, .len = sizeof(ch16) },
{ .rx_buf = rx16, .len = sizeof(rx16) },
};
spi_transfer(spi, rdid, ARRAY_SIZE(rdid));
Since you have a scope, be sure to check that this operation happens under a single chip select pulse. I have found more than one Linux SPI driver to have a bug that pulses chip select when it should not. In some cases switching from TX to RX (like done above) will trigger a CS pulse. In other cases a CS pulse is generated for every word (8 bits here) of data.
Another thing you should change is use dev_info(&spi->dev, "device version %d", id)' and also dev_err() to print messages. This inserts the device name in a standard way instead of your hard-coded non-standard and inconsistent "[FRAME DRIVER]::" text. And sets the level of the message as appropriate.
Also, consider supporting device tree in your driver to read device properties. Then you can do things like change the SPI bus frequency for this device without rebuilding the kernel driver.