Can I select() on a /dev/spidev file descriptor?

Question

I'm maintaining some userspace code that talks to a FPGA via SPI. Right now it's polling to see if there's data to act on, which I'm not thrilled about. The (heavily-simplified) structure of the comm thread looks like this:

int spi_fd;

void do_transfer(char *buf, int len)
{
    struct spi_ioc_transfer xfer;
    memset(xfer, 0, sizeof(xfer));

    ioctl_tell_some_fpga_register_heads_up();

    xfer[0].len = len;
    xfer[0].tx_buf = NULL;
    xfer[0].rx_buf = buf;
    ioctl(spi_fd, SPI_IOC_MESSAGE(1), xfer);

    ioctl_tell_some_fpga_register_were_done();
}

void *comm_thread(void arg)
{
    uint8_t config = SPI_MODE_3;
    __u32 speed = 4000000;
    char buffer[5120];

    spi_fd = open("/dev/spidev1.0", O_RDWR);
    ioctl(spi_fd, SPI_IOC_WR_MODE, &config);
    ioctl(spi_fd, SPI_IOC_WR_MAX_SPEED_HZ, &speed);

    while(1) {
        sleep(2); //ugh
        if(ioctl_read_some_fpga_register_that_says_if_theres_data())
        {
            do_transfer(buffer, some_small_number_of_bytes());
            do_stuff(buffer); //you get the picture
        }
    }
}

I'd really prefer an event-based solution over poll-and-sleep. The first thing that came to mind was doing a select() on the spidev file descriptor instead of checking some register every X seconds, something like

fd_set myset;

while(1) {
    FD_ZERO(&myset);
    FD_SET(spi_fd, &myset);
    select(spi_fd + 1, &myset, NULL, NULL, NULL);
    do_transfer(buffer, some_small_number_of_bytes());
    do_stuff(buffer);
}

Thing is I can't find any examples of people handling SPI like that, and I'm wondering if maybe there's a good reason for it. Can /dev/spidev be used this way? Will it do something goofy like always/never being "ready to read"? Can it be made to behave the way I want? Is it hardware dependent? I'm not averse to a little kernel driver hacking if it's necessary, but I'm not really sure if/where I need to be looking.

Answer 1

Can I select() on a /dev/spidev file descriptor?

No.
The spidev documentation states

At this time there is no async I/O support; everything is purely synchronous.

More importantly the spidev driver does not support a poll file operation. The select() syscall requires the device driver to support a poll fops.

670 static const struct file_operations spidev_fops = {
671         .owner =        THIS_MODULE,
672         /* REVISIT switch to aio primitives, so that userspace
673          * gets more complete API coverage.  It'll simplify things
674          * too, except for the locking.
675          */
676         .write =        spidev_write,
677         .read =         spidev_read,
678         .unlocked_ioctl = spidev_ioctl,
679         .compat_ioctl = spidev_compat_ioctl,
680         .open =         spidev_open,
681         .release =      spidev_release,
682         .llseek =       no_llseek,
683 };