/* [cgw]: 取出spi_message */
m = container_of(bitbang->queue.next, struct spi_message,
queue);
/* [cgw]: 删除这个节点 */
list_del_init(&m->queue);
/* [cgw]: 进入临界区 */
spin_unlock_irqrestore(&bitbang->lock, flags);

/* FIXME this is made-up ... the correct value is known to
* word-at-a-time bitbang code, and presumably chipselect()
* should enforce these requirements too
*/
nsecs = 100;

spi = m->spi;
tmp = 0;
cs_change = 1;
status = 0;
setup_transfer = NULL;

/* [cgw]: 历遍spi_message中的收发列表 */
list_for_each_entry (t, &m->transfers, transfer_list) {

/* override or restore speed and wordsize */
/* [cgw]: 如果驱动指定了spi速度，和位数，重新调用spi_bitbang_setup_transfer
* 更改默认设置
*/
if (t->speed_hz || t->bits_per_word) {
setup_transfer = bitbang->setup_transfer;
if (!setup_transfer) {
status = -ENOPROTOOPT;
break;
}
}
if (setup_transfer) {
status = setup_transfer(spi, t);
if (status < 0)
break;
}

/* set up default clock polarity, and activate chip;
* this implicitly updates clock and spi modes as
* previously recorded for this device via setup().
* (and also deselects any other chip that might be
* selected ...)
*/
/* [cgw]: 激活spi */
if (cs_change) {
bitbang->chipselect(spi, BITBANG_CS_ACTIVE);
ndelay(nsecs);
}
/* [cgw]: 驱动指定收发完一帧数据要不要改变恢复CS为空闲 */
cs_change = t->cs_change;
/* [cgw]: 收发包为空，则无效 */
if (!t->tx_buf && !t->rx_buf && t->len) {
status = -EINVAL;
break;
}

/* transfer data. the lower level code handles any
* new dma mappings it needs. our caller always gave
* us dma-safe buffers.
*/
if (t->len) {
/* REVISIT dma API still needs a designated
* DMA_ADDR_INVALID; ~0 might be better.
*/
if (!m->is_dma_mapped)
t->rx_dma = t->tx_dma = 0;
/* [cgw]: 这里才是真正的实现spi收发时序 */
status = bitbang->txrx_bufs(spi, t);
}
if (status != t->len) {
if (status > 0)
status = -EMSGSIZE;
break;
}
m->actual_length += status;
status = 0;

/* protocol tweaks before next transfer */
if (t->delay_usecs)
udelay(t->delay_usecs);

/* [cgw]: 收发完一帧，不改变CS状态 */
if (!cs_change)
continue;

/* [cgw]: 收发列表已经没有数据，结束 */
if (t->transfer_list.next == &m->transfers)
break;

/* sometimes a short mid-message deselect of the chip
* may be needed to terminate a mode or command
*/
/* [cgw]: 释放spi */
ndelay(nsecs);
bitbang->chipselect(spi, BITBANG_CS_INACTIVE);
ndelay(nsecs);
}

m->status = status;
m->complete(m->context);

/* restore speed and wordsize */
/* [cgw]: 速度和位数恢复默认 */
if (setup_transfer)
setup_transfer(spi, NULL);