The bulk of the driver will be managing the I/O queue fed by transfer().
That queue could be purely conceptual. For example, a driver used only
-for low-frequency sensor acess might be fine using synchronous PIO.
+for low-frequency sensor access might be fine using synchronous PIO.
But the queue will probably be very real, using message->queue, PIO,
often DMA (especially if the root filesystem is in SPI flash), and