[PATCH 0/4] serial: 8250: Fixes for Oxford Semiconductor 950 UARTs

Maciej W. Rozycki
2021-06-10T18:38:58Z

Hi,

In the course of verifying support for SMSC FDC37M817 Super I/O chip's
data rate of 460800bps with an EXSYS EX-44072 option card based on the
Oxford Semiconductor OXPCIe952 device I have observed the inability to
transfer any data correctly at this rate between the two devices. Lower
rates, including 230400bps, appeared to work correctly, also with other
kinds of serial ports referred to with my previous patch series, which
strongly indicated something being wrong with the Oxford device.

In the end I have tracked the issue down to our baud base set to 4000000
for the device being off by 2.4%. Enough for an incorrect divisor value
of 9 to be chosen to yield the bit rate of 460800bps being particularly
inaccurate for the baud base selected and caused the actual rate of
434027.78bps of being used, -5.8% off. Consequently the ten bits of data
sent with every 8-bit character were enough for the drift to accumulate up
to the two ends to get out of sync, with the stop bit interpreted as bit 7
of data. Obviously whoever wrote this code never actually tried higher
data rates, or only connected Oxford devices to each other causing the
systematic errors at both ends to cancel each other.

With the baud base corrected to the chip's default of 3906250 for the 650
mode which we use (the reset default for the initial 450 mode is 115314,
approximated for maximum backwards compatibility with legacy OS drivers by
dividing the device's 62.5MHz clock by 33.875), the new calculated divisor
value and the new actual bit rate became 8 and 488281.25bps respectively.
Now +5.96% off, so the stop bit could be missed causing data corruption
with continuous data streams, but at least that could be worked around by
using two stop bits instead. Not a good solution though.

So I chose to implement proper clock handling for the chip. The bit rate
with this device is worked out from the 62.5MHz clock first by choosing an
oversampling rate between 4 and 16 inclusive, then by the clock prescaler
between 1 and 63.875 in increments of 0.125, and finally a 16-bit unsigned
divisor, all of which divide the input clock by the respective value.

By choosing the right values of these three parameters either exact or
highly-accurate actual bit rates can be programmed for standard and many
non-standard rates from 1bps up to 15625000bps, e.g. for the data rate of
460800bps concerned here I was able to get the accuracy of 0.0064% by
choosing the values of 7, 3.875, and 5 respectively for the oversampling
rate, the clock prescaler, and the clock divisor.

Additionally even with my considerably mighty POWER9 box I have observed
frequent input overruns with the bit rates of 460800bps and higher, and I
have noticed we have the receive interrupt trigger level set particularly
high in terms of FIFO usage percentage for 16C950 UARTs and then we don't
make the levels configurable. Lowering the default to a saner value made
the overruns go away altogether for rates below 921600bps. As I've only
verified these changes in terminal environment rather than with modems I
could not make use of hardware flow control which this chip supports and
which I presume would prevent overruns from happening even with higher bit
rates.

There's more that could be done here, for example we don't make use of
the 950 mode where FIFO trigger levels can be fine-tuned in increments of
1, which, interestingly, could help with the lower rate modes as reception
is quite choppy with them, owing to the minimum receive interrupt trigger
level of 16 in the 650 mode. I gave the 950 mode a try, but it made the
chip freeze frequently until new data was received, so clearly I must have
missed something in the chip's configuration, which I did not investigate.
Something for a different time perhaps then.

I have verified these changes for standard termios rates between 300bps
and 460800bps with my WTI CPM-800 site manager device and my Malta board's
serial ports, as suitable, all working flawlessly in terminal mode now.
I have verified standard termios rates between 500000bps and 4000000bps as
well, however for the lack of other high-speed hardware with a pair of
Oxford devices only. Except for input overruns noted above and growing in
numbers as the rate increased rates of up to 3500000bps worked flawlessly.
In particular the rate of 576000bps, still without input overruns, gave
this nice feeling as if working with a virtual terminal rather than over a
serial line!

Conversely the rate of 4000000bps showed significant data corruption,
happening randomly, i.e. some characters went through just fine, while
other ones became garbled in no particular pattern, unlike with the rate
inaccuracy described above. Also with no input overruns whatsoever. I
have double-checked that all the three parameters making up the bit rate
from the clock rate have been programmed correctly.

Therefore I have concluded this is not an issue with my change (or indeed
any other part the driver) and it is simply that the rate has exceeded
either the maximum frequency the EX-44072 board's serial transceivers
support (I haven't checked the chip types used and I can't persuade myself
to disassemble the system just to have a look at the board again), or the
bandwidth of the transmission line used (a flat 8-core telephone cable of
a standard Cisco console cable assembly). Not an issue to be addressed in
software and I find it rather astonishing anyway it worked so well for up
to 3.5MHz already!

I have no modems, so I couldn't verify DCE interoperation, but I don't
expect issues with the bit rates being more accurate now, or the default
FIFO receiver trigger level tweaked to be more conservative.

Finally the 16-bit UART_DIV_MAX limitation of the baud rate requested
with `serial8250_get_baud_rate' makes the standard rates of 200bps and
lower inaccessible in the regular way with the baud base of 15625000.
That could be avoided by tweaking our 8250 driver core appropriately, but
I have figured out with modern serial port usage that would not be the
best use of my time. Someone who does have a real need to use an Oxford
device at these low rates can step in and make the necessary chances.

Meanwhile I took advantage of the ancient spd_cust feature we thankfully
continue supporting and actually did verify not only the standard rates
between 50bps and 200bps, but the rates of 4bps and 2bps as well, using my
old x86 server's serial port with the baud base of 115200. That was,
ahem, an interesting experience both by itself and also with 2bps, which
revealed a phenomenon with SMSC Super I/O ports not working as documented
(already noted in the preceding patch series). Eventually I verified the
2bps rate with a plain ISA multi I/O card and its 16450 UART my EISA 486
box has as the remote console, which does support the divisor value of
57600 required.

See individual change descriptions for further details including figures.

Please apply.

Maciej