Hi, On Tue, Oct 01, 2013 at 10:18:08AM -0700, Tim Kryger wrote: > When configured with UART_16550_COMPATIBLE=NO or in versions prior to > the introduction of this option, the Designware UART will ignore writes > to the LCR if the UART is busy. The current workaround saves a copy of > the last written LCR and re-writes it in the ISR for a special interrupt > that is raised when a write was ignored. > > Unfortunately, interrupts are typically disabled prior to performing a > sequence of register writes that include the LCR so the point at which > the retry occurs is too late. An example is serial8250_do_set_termios() > where an ignored LCR write results in the baud divisor not being set and > instead a garbage character is sent out the transmitter. > > Furthermore, since serial_port_out() offers no way to indicate failure, > a serious effort must be made to ensure that the LCR is actually updated > before returning back to the caller. This is difficult, however, as a > UART that was busy during the first attempt is likely to still be busy > when a subsequent attempt is made unless some extra action is taken. > > This updated workaround reads back the LCR after each write to confirm > that the new value was accepted by the hardware. Should the hardware > ignore a write, the TX/RX FIFOs are cleared and the receive buffer read > before attempting to rewrite the LCR out of the hope that doing so will > force the UART into an idle state. While this may seem unnecessarily > aggressive, writes to the LCR are used to change the baud rate, parity, > stop bit, or data length so the data that may be lost is likely not > important. Admittedly, this is far from ideal but it seems to be the > best that can be done given the hardware limitations. > > Lastly, the revised workaround doesn't touch the LCR in the ISR, so it > avoids the possibility of a "serial8250: too much work for irq" lock up. > This problem is rare in real situations but can be reproduced easily by > wiring up two UARTs and running the following commands. > > # stty -F /dev/ttyS1 echo > # stty -F /dev/ttyS2 echo > # cat /dev/ttyS1 & > [1] 375 > # echo asdf > /dev/ttyS1 > asdf > > [ 27.700000] serial8250: too much work for irq96 > [ 27.700000] serial8250: too much work for irq96 > [ 27.710000] serial8250: too much work for irq96 > [ 27.710000] serial8250: too much work for irq96 > [ 27.720000] serial8250: too much work for irq96 > [ 27.720000] serial8250: too much work for irq96 > [ 27.730000] serial8250: too much work for irq96 > [ 27.730000] serial8250: too much work for irq96 > [ 27.740000] serial8250: too much work for irq96 > > Signed-off-by: Tim Kryger <tim.kryger@xxxxxxxxxx> > Reviewed-by: Matt Porter <matt.porter@xxxxxxxxxx> > Reviewed-by: Markus Mayer <markus.mayer@xxxxxxxxxx> > --- > > Changes in v2: > - Rebased on tty-next > - Updated commit messsage to mention UART_16550_COMPATIBLE > - Removed potentially unnecessary read of LSR and MSR > - Only attempt workaround when LCR write is ignored I'm OK with this. Reviewed-by: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx> Br, -- heikki -- 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
