On 06/14/2013 12:29 PM, Grant Edwards wrote:
On 2013-06-14, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
On 06/14/2013 11:17 AM, Grant Edwards wrote:
On 2013-06-14, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
On 06/12/2013 04:03 PM, Grant Edwards wrote:
I see the uart_ops.flush_buffer method which is used to flush the
UART's tx fifo (presumably when the user calls tcflush(TCOFLUSH)).
How does the rx fifo get flushed when the user calls tcflush(TCIFLUSH)?
It doesn't.
Thanks. I couldn't see any mechanism to do that, and I thought I must
be missing something.
If you're seeing stale i/o, it's more likely due to the flip buffers
not being flushed
Probably. There is a scenario where you can get old data because the
rx fifo isn't flushed, but I suspect it's not what my customer is
complaining about. FWIW, here's the scenario I'm worrying about:
1) Enable either RTS/CTS or Xon/Xoff flow control for a UART driver
that handles that flow control in hardware[1].
2) Stop making read() calls on the tty device.
3) The buffers in the tty layer fill up, so the uart driver stops
transferring data from the rx fifo to the tty layer.
4) The rx fifo fills up, and the flow control stops the other end
from sending data.
[all working OK up to this point, now you wait for an arbitrary
amount of time]
5) tcflush(TCIFLUSH) is called.
[data in the tty layer gets flushed, but old data in the rx
fifo remains]
Yep. Your driver continues to push new data as it should, but that's
getting buffered up in the flip buffers. So that is what the app is
reading now that it has restarted read()s.
Your hardware rx fifo shouldn't have stale data in it because that
should generate an overrun; ie., if the flip buffers cannot accept
data because they're full then the next char pushed when space
becomes available should be a NUL flagged with TTY_OVERRUN.
If flow control is enabled, there should be no rx overruns -- that's
what flow control is for. In the scenario above, flow control is
enabled (and working). In order to allow the UART to handle flow
control, the UART driver must stop reading data from the rx fifo when
the tty layer is "full". The documentation for the serial core API
specifically states that UARTs are allowed to implement flow control
in hardware, and the only way that can be done is to alow the rx fifo
to fill up when the application stops makeing read() calls and the tty
layer fills up.
I think in newer kernels instead of explicitly checking for room in
the tty layer before unloading the rx fifo, the UART is supposed to
rely on the throttle/unthrottle callbacks, but the end result is the
same: when the tty layer gets "full", the UART driver stops reading
data from the rx fifo, and the rx fifo fills up.
AFAIK, only USB serial stops reading the rx fifo on throttle;
the serial core and other tty drivers continue to empty the rx fifo --
throttle only shuts off the transmitter on the other end.
Without handling throttle/unthrottle, how are you determining that the
tty layer is "full"? Return code from tty_insert_flip_xxxx()?
Regards,
Peter Hurley
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