• Marcel Moolenaar's avatar
    The uart(4) driver is an universal driver for various UART hardware. · 27d5dc18
    Marcel Moolenaar authored
    It improves on sio(4) in the following areas:
    o  Fully newbusified to allow for memory mapped I/O. This is a must
       for ia64 and sparc64,
    o  Machine dependent code to take full advantage of machine and firm-
       ware specific ways to define serial consoles and/or debug ports.
    o  Hardware abstraction layer to allow the driver to be used with
       various UARTs, such as the well-known ns8250 family of UARTs, the
       Siemens sab82532 or the Zilog Z8530. This is especially important
       for pc98 and sparc64 where it's common to have different UARTs,
    o  The notion of system devices to unkludge low-level consoles and
       remote gdb ports and provides the mechanics necessary to support
       the keyboard on sparc64 (which is UART based).
    o  The notion of a kernel interface so that a UART can be tied to
       something other than the well-known TTY interface. This is needed
       on sparc64 to present the user with a device and ioctl handling
       suitable for a keyboard, but also allows us to cleanly hide an
       UART when used as a debug port.
    Following is a list of features and bugs/flaws specific to the ns8250
    family of UARTs as compared to their support in sio(4):
    o  The uart(4) driver determines the FIFO size and automaticly takes
       advantages of larger FIFOs and/or additional features. Note that
       since I don't have sufficient access to 16[679]5x UARTs, hardware
       flow control has not been enabled. This is almost trivial to do,
       provided one can test. The downside of this is that broken UARTs
       are more likely to not work correctly with uart(4). The need for
       tunables or knobs may be large enough to warrant their creation.
    o  The uart(4) driver does not share the same bumpy history as sio(4)
       and will therefore not provide the necessary hooks, tweaks, quirks
       or work-arounds to deal with once common hardware. To that extend,
       uart(4) supports a subset of the UARTs that sio(4) supports. The
       question before us is whether the subset is sufficient for current
    o  There is no support for multiport UARTs in uart(4). The decision
       behind this is that uart(4) deals with one EIA RS232-C interface.
       Packaging of multiple interfaces in a single chip or on a single
       expansion board is beyond the scope of uart(4) and is now mostly
       left for puc(4) to deal with. Lack of hardware made it impossible
       to actually implement such a dependency other than is present for
       the dual channel SAB82532 and Z8350 SCCs.
    The current list of missing features is:
    o  No configuration capabilities. A set of tunables and sysctls is
       being worked out. There are likely not going to be any or much
       compile-time knobs. Such configuration does not fit well with
       current hardware.
    o  No support for the PPS API. This is partly dependent on the
       ability to configure uart(4) and partly dependent on having
       sufficient information to implement it properly.
    As usual, the manpage is present but lacks the attention the
    software has gotten.