On 9/10/21 9:35 PM, Hinko Kocevar wrote: > I have an emulated MMIO area holding couple of registers that deal with > serial UART. Very simple access to the Tx and Rx registers from the > userspace point of view involves polling for a bit in one register and > then writing another; when there is room for another character. When the > guest app does write to a MMIO Tx register, as expected, io_writex() is > invoked and my handler is invoked. At the moment it does not do much. > I'm thinking now that the character needs to be fed to the serial device > instance or something. > > Where should I look for suitable examples in the qemu code? I reckon > that other machines exist that do the similar. I found lots of > serial_mm_init() and sysbus_mmio_map() uses around serial port instances > but I'm not sure how to couple my "serial ops" to the "bus" or SerialMM > (if that is the way to go).
Your device is a "character device frontend". See the API in include/chardev/char-fe.h. Frontends can be connected to various backends. The simplest backend is the standard input/output (named 'stdio'). To be useful your frontend have to implement some handlers: IOEventHandler, IOCanReadHandler, IOReadHandler. The frontend register these handlers by calling qemu_chr_fe_set_handlers() (usually in the DeviceRealize() handler). The backends will interact with your device via this API. I recommend you to look at the hw/char/digic-uart.c model which is quite simple, it returns the last char received, and only transmit one char per I/O. Then for a more complete (and up to date) model you can look at hw/char/goldfish_tty.c, it uses a FIFO to receive chars, but still transmit one char at a time. Finally the hw/char/serial.c is probably the most complete models, with 2 FIFOs (RX & TX) and try to respect timings. Regards, Phil.
