spi_bitbang_start

/**
 * spi_bitbang_start - start up a polled/bitbanging SPI master driver
 * @bitbang: driver handle
 *
 * Caller should have zero-initialized all parts of the structure, and then
 * provided callbacks for chip selection and I/O loops.  If the master has
 * a transfer method, its final step should call spi_bitbang_transfer; or,
 * that's the default if the transfer routine is not initialized.  It should
 * also set up the bus number and number of chipselects.
 *
 * For i/o loops, provide callbacks either per-word (for bitbanging, or for
 * hardware that basically exposes a shift register) or per-spi_transfer
 * (which takes better advantage of hardware like fifos or DMA engines).
 *
 * Drivers using per-word I/O loops should use (or call) spi_bitbang_setup,
 * spi_bitbang_cleanup and spi_bitbang_setup_transfer to handle those spi
 * master methods.  Those methods are the defaults if the bitbang->txrx_bufs
 * routine isn't initialized.
 *
 * This routine registers the spi_master, which will process requests in a
 * dedicated task, keeping IRQs unblocked most of the time.  To stop
 * processing those requests, call spi_bitbang_stop().
 */
int spi_bitbang_start(struct spi_bitbang *bitbang)
{
	struct spi_master *master = bitbang->master;
	int status;

	if (!master || !bitbang->chipselect)
		return -EINVAL;

	INIT_WORK(&bitbang->work, bitbang_work);
	spin_lock_init(&bitbang->lock);
	INIT_LIST_HEAD(&bitbang->queue);

	if (!master->mode_bits)
		master->mode_bits = SPI_CPOL | SPI_CPHA | bitbang->flags;

	if (!master->transfer)
		master->transfer = spi_bitbang_transfer;
	if (!bitbang->txrx_bufs) {
		bitbang->use_dma = 0;
		bitbang->txrx_bufs = spi_bitbang_bufs;
		if (!master->setup) {
			if (!bitbang->setup_transfer)
				bitbang->setup_transfer =
					 spi_bitbang_setup_transfer;
			master->setup = spi_bitbang_setup;
			master->cleanup = spi_bitbang_cleanup;
		}
	} else if (!master->setup)
		return -EINVAL;
	if (master->transfer == spi_bitbang_transfer &&
			!bitbang->setup_transfer)
		return -EINVAL;

	/* this task is the only thing to touch the SPI bits */
	bitbang->busy = 0;
	bitbang->workqueue = create_singlethread_workqueue(
			dev_name(master->dev.parent));
	if (bitbang->workqueue == NULL) {
		status = -EBUSY;
		goto err1;
	}

	/* driver may get busy before register() returns, especially
	 * if someone registered boardinfo for devices
	 */
	status = spi_register_master(master);
	if (status < 0)
		goto err2;

	return status;

err2:
	destroy_workqueue(bitbang->workqueue);
err1:
	return status;
}

对spi_master结构体的mode_bits、transfer、setup、cleanup设置，还有spi_bitbang结构体设置，最后用spi_register_master注册spi_master.