Re: simple serial device emulation

From: Peter Maydell <peter.maydell@linaro.org>
To: "Philippe Mathieu-Daudé" <f4bug@amsat.org>
Cc: Hinko Kocevar <hinkocevar@gmail.com>,
	QEMU Developers <qemu-devel@nongnu.org>
Subject: Re: simple serial device emulation
Date: Sat, 11 Sep 2021 16:23:25 +0100	[thread overview]
Message-ID: <CAFEAcA8DHC+_2BqqdH88oVDoNuPGuWysMC3DjDt4oj2_wfAKLg@mail.gmail.com> (raw)
In-Reply-To: <fe2a2b9d-e7b6-cbf6-ea3a-49702e297562@amsat.org>

On Fri, 10 Sept 2021 at 22:51, Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
>
> 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').

More specifically, it's a UART model. All of our UART models
are in hw/char/.

> 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.

digic-uart does still use the old qemu_chr_fe_write_all() blocking
API, though (there is an XXX comment about that). If you want an
example of the non-blocking approach, try hw/char/cmsdk-apb-uart.c.

> Finally the hw/char/serial.c is probably the most complete models,
> with 2 FIFOs (RX & TX) and try to respect timings.

hw/char/serial.c is kind of complicated though, both because
it's quite old code that's been gradually modernized, and also
because it has to support both mmio and io port type serial ports.
So I'm not sure I'd recommend it as an example to learn from.

-- PMM