an UART interface could get stuck when a new interrupt condition
arose while servicing a previous interrupt. Since an interrupt was
already pending, no new interrupt would be triggered.
Avoid infinite recursion by flushing the Rx FIFO and marking an
overrun condition when we could not move the data from the Rx
FIFO to the receive buffer in toto. Failure to flush the Rx FIFO
would leave the Rx ready condition pending.
Note that the SAB 82532 already did this due to the nature of the
chip.
precisely where locking would be needed before adding it, but it
seems uart(4) draws slightly too much attention to have it without
locking for too long.
The lock added is a spinlock that protects access to the underlying
hardware. As a first and obvious stab at this, each method of the
hardware interface grabs the lock. Roughly speaking this serializes
the methods. Exceptions are the probe, attach and detach methods.
We simply use the detected FIFO size to determine whether we have
a post 16550 UART or not. The support lacks proper serialization of
hardware access for now.
is not a size of 1. Since we already know there is a FIFO, we can
safely assume that it is at least 16 bytes. Note that all this is
mostly academic anyway. We don't use the size of the Rx FIFO
currently. If we add support for hardware flow control, we only
care about Rx FIFO sizes larger than 16.
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
hardware.
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.
