Dear All, I'm having trouble understanding the difference between spi_sync and spi_async: Theoretically, spi_sync waits for completion while spi_async provides completion callbacks. However, on my platform spi_async does not do anything! I have the following code that is supposed to write a single byte. Using spi_sync works fine, calling the completion callback with spi_async also works. I just do not see anything on the connected oscilloscope screen. What am I missing? Thanks in advance Moritz int lprf_set_WREN(void) { int status, ret; int sync = 0; struct spi_message m; struct spi_transfer t; u8 *buffer = kmalloc(sizeof(u8), GFP_KERNEL); memset(buffer, 0, sizeof(u8)); buffer[0] = 0x06; t.tx_buf = buffer; t.rx_buf = NULL; printk(KERN_DEBUG "lprf: buffer=%p\n", buffer); t.len = 1; t.bits_per_word = 0; spi_message_init(&m); spi_message_add_tail(&t, &m); printk(KERN_DEBUG "lprf: lprf_set_WREN start\n"); if (sync) { /* Variante A: sync */ status = spi_sync(lp->spi, &m); printk(KERN_DEBUG "lprf: lprf_set_WREN sync end. status=%d \n", status); } else { /* Variante B: async */ init_completion(&lp->state_complete); m.complete = lprf_mycomplete; m.context = &lp->state_complete; ret = spi_myvalidate(lp->spi, &m); if (ret) { printk(KERN_DEBUG "spi_myvalidate returned %d\n", ret); return ret; } status = spi_async_locked(lp->spi, &m); if (status == 0) { wait_for_completion(&lp->state_complete); status = m.status; } printk(KERN_DEBUG "lprf: lprf_set_WREN async end status=%d m.status=%d\n", status, m.status); m.context = NULL; } if (status) printk(KERN_ERR "lprf: spi_async failed at %s, %i\n", __FILE__, __LINE__); return status; }