QEMU CHARACTER "DEVICES" MANAGEMENT

I. CharDriverState objects:
---------------------------

One of the strangest abstraction in QEMU is the "CharDriverState"
(abbreviated here as "CS").

The CS is essentially an object used to model a character stream that
can be connected to things like a host serial port, a host network socket,
an emulated device, etc...

What's really unusual is its interface though, which comes from the fact
that QEMU implements a big event loop with no blocking i/o allowed. You
can see "qemu-char.h" for the full interface, but here we will only describe
a few important functions:

  - qemu_chr_write() is used to try to write data into a CS object. Note that
    success is not guaranteed: the function returns the number of bytes that
    were really written (which can be 0) and the caller must deal with it.
    This is very similar to writing to a non-blocking BSD socket on Unix.

       int  qemu_chr_write( CharDriverState*  cs,
                            const uint8_t*    data,
                            int               datalen );

    This function may return -1 in case of error, but this depends entirely
    on the underlying implementation (some of them will just return 0 instead).
    In practice, this means it's not possible to reliably differentiate between
    a "connection reset by peer" and an "operation in progress" :-(

    There is no way to know in advance how many bytes a given CharDriverState
    can accept, nor to be notified when its underlying implementation is ready
    to accept data again.


  - qemu_chr_add_handler() is used to add "read" and "event" handlers
    to a CS object. We will ignore "events" here and focus on the
    "read" part.

    Thing is, you cannot directly read from a CS object. Instead, you provide
    two functions that will be called whenever the object has something for
    you:

        - a 'can_read' function that shall return the number of bytes
          that you are ready to accept from the CharDriverState. Its
          interface is:

             typedef int  IOCanRWHandler (void*  opaque);

        - a 'read' function that will send you bytes from the CharDriverState

             typedef void IOReadHandler  (void*           opaque,
                                          const uint8_t*  data,
                                          int             datalen);

          normally, the value of 'datalen' cannot be larger than the result
          of a previous 'can_read' call.

    For both callbacks, 'opaque' is a value that you pass to the function
    qemu_chr_add_handler() which signature is:

         void qemu_chr_add_handlers(CharDriverState *s,
                                    IOCanRWHandler  *fd_can_read,
                                    IOReadHandler   *fd_read,
                                    IOEventHandler  *fd_event,
                                    void            *opaque);

  - qemu_chr_open() is used to create a new CharDriverState object from a
    descriptive string, its interface is:

         CharDriverState*  qemu_chr_open(const char*  filename);

    there are various formats for acceptable 'filenames', and they correspond
    to the parameters of the '-serial' QEMU option described here:

       http://www.nongnu.org/qemu/qemu-doc.html#SEC10

    For example:

       "/dev/<file>" (Linux and OS X only):
            connect to a host character device file (e.g. /dev/ttyS0)

       "file:<filename>":
            Write output to a given file (write only)

       "stdio":
            Standard input/output

       "udp:[<remote_host>]:<remote_port>[@[<src_ip>]:<src_port>]":
            Connect to a UDP socket for both read/write.

       "tcp:[<host>]:<port>[,server][,nowait][,nodelay]"
            Connect to a TCP socket either as a client or a server.

            The 'nowait' option is used to avoid waiting for a client
            connection.

            The 'nodelay' is used to disable the TCP Nagle algorithm to
            improve throughput.

    For Android, a few special names have been added to the internal
    implementation and redirect to program functions:

       "android-kmsg":
            A CharDriverState that is used to receive kernel log messages
            from the emulated /dev/ttyS0 serial port.

       "android-qemud":
            A CharDriverState that is used to exchange messages between the
            emulator program and the "qemud" multiplexing daemon that runs in
            the emulated system.

            The "qemud" daemon is used to allow one or more clients in the
            system to connect to various services running in the emulator
            program. This is mainly used to bypass the kernel in order to
            implement certain features with ease.

            (see docs/ANDROID-QEMUD.TXT for details)

       "android-gsm":
            A CharDriverState that is used to connect the emulated system to
            a host modem device with the -radio <device> option. Otherwise,
            the system uses qemud to connect to the emulator's internal modem
            emulation.

        "android-gps":
            A CharDriverState that is used to connect the emulated system to a
            host GPS device with the -gps <device> option. Otherwise the
            system uses qemud to connect to the emulator's internal GPS
            emulation.


II. CharDriverState users:
--------------------------

As described above, a CharDriverState "user" is a piece of code that can write
to a CharDriverState (by calling qemu_chr_write() explicitely) and can also
read from it after registering can_read/read handlers for it through
qemu_chr_add_handlers().

Typical examples are the following:

  - The hardware serial port emulation (e.g. hw/android/goldfish/tty.c) will
    read data from the kernel then send it to a CS. It also uses a small buffer
    that is used to read data from the CS and send it back to the kernel.

  - The Android emulated modem also uses a CS to talk with its client,
    which will in most cases be an emulated serial port.


III. CharBuffer objects:
------------------------

The Android emulator provides an object called a CharBuffer which acts as
a CharDriverState object that implements a *write* buffer to send data to a
given CS object, called the endpoint. You can create one with:

    #include "charpipe.h"
    CharDriverState*  qemu_chr_open_buffer( CharDriverState*  endpoint );

This function returns a new CS object that will buffer in the heap any data
that is sent to it, but cannot be sent to the endpoint yet. On each event loop
iteration, the CharBuffer will try to send data to the endpoint until it
doesn't have any data left.

This can be useful to simplify certain CS users who don't want to maintain
their own emit buffer. Note that writing to a CharBuffer always succeeds.

Note also that calling qemu_chr_add_handler() on the CharBuffer will do the
same on the endpoint. Any endpoint-initiated calls to can_read()/read()
callbacks are passed directly to your handler functions.


IV. CharPipe objects:
---------------------

The Android emulator also provides a convenient abstraction called a "charpipe"
used to connect two CharDriverState users together. For example, this is used
to connect a serial port emulation (in hw/android/goldfish/tty.c) to the
internal GSM modem emulation (see telephony/modem_driver.c).

Essentially, a "charpipe" is a bi-directionnal communication pipe whose two
endpoints are both CS objects. You call "qemu_chr_open_pipe()" to create the
pipe, and this function will return the two endpoints to you:

    #include "charpipe.h"
    int  qemu_chr_open_pipe(CharDriverState* *pfirst,
                            CharDriverState* *psecond);

When you write to one end of the pipe (with qemu_chr_write()), the charpipe will
try to write as much data as possible to the other end. Any remaining data is stored
in a heap-allocated buffer.

The charpipe will try to re-send the buffered data on the next event loop
iteration by calling the can_read/read functions of the corresponding user,
if there is one.

Note that there is no limit on the amount of data buffered in a charpipe,
and writing to it is never blocking. This simplifies CharDriverState
users who don't need to worry about buffering issues.