Happy Holiday Guys!

I m learning SPI interface.

static ssize_t
spidev_write(struct file *filp, const char __user *buf,
size_t count, loff_t *f_pos)
{
struct spidev_data *spidev;
ssize_t status = 0;
unsigned long missing;

/* chipselect only toggles at start or end of operation */
if (count > bufsiz)
return -EMSGSIZE;

spidev = filp->private_data;

mutex_lock(&spidev->buf_lock);
missing = copy_from_user(spidev->buffer, buf, count);
if (missing == 0) {
status = spidev_sync_write(spidev, count);
} else
status = -EFAULT;
mutex_unlock(&spidev->buf_lock);

return status;
}


static inline ssize_t
spidev_sync_write(struct spidev_data *spidev, size_t len)
{
struct spi_transfer t = {
.tx_buf = spidev->buffer,
.len = len,
};
struct spi_message m;

spi_message_init(&m);
spi_message_add_tail(&t, &m); //To add transfers to a spi_message, use the spi_message_add_tail() function
return spidev_sync(spidev, &m); // The core method for submitting any message to the SPI system is spi_async()
}

static ssize_t
spidev_sync(struct spidev_data *spidev, struct spi_message *message)
{
DECLARE_COMPLETION_ONSTACK(done);
int status;

message->complete = spidev_complete;
message->context = &done;

spin_lock_irq(&spidev->spi_lock);
if (spidev->spi == NULL)
status = -ESHUTDOWN;
else
status = spi_async(spidev->spi, message);
spin_unlock_irq(&spidev->spi_lock);

if (status == 0) {
wait_for_completion(&done);
status = message->status;
if (status == 0)
status = message->actual_length;
}
return status;
}


In the write function of spidev.c, it has "copy_from_user" and "spidev_sync_write(spidev, count) ->spidev_sync(spidev, &m)-> spi_async(spidev->spi, message) ". Both of two functions are sending message from user to kernel(SPI device), why the write function implement both of them? What's the difference between "copy_from_user" and "spi_async(spidev->spi, message)"