On Fri, Jun 14, 2013 at 01:09:56PM -0400, Peter Hurley wrote: > On 06/14/2013 12:29 PM, Grant Edwards wrote: > > >> 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; All the drivers I maintain do that. It's the only way to get flow control to work. For UART with large FIFOs (e.g. 1KB) -- espcially those attached via USB or Ethernet -- flow control driven by code in serial_core just doesn't work right: you've got to let the UART handle it. > 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()? I check tty->receive_room. What are you supposed to do for kernel versions that don't have the throttle()/unthrottle() callbacks? -- Grant -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
