On Tue, Sep 24, 2013 at 05:39:09PM -0700, Tim Kryger wrote: > The Designware UART has a limitation where it ignores writes into the > LCR if the UART is busy. The current workaround stashes a copy of the > last written LCR and writes it back down to the hardware if it receives > a special busy interrupt which 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 takes the extreme action of clearing the TX/RX > FIFOs and reading the receive buffer before writing down the LCR in 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> > --- > drivers/tty/serial/8250/8250_dw.c | 57 +++++++++++++++++++++++++++------------ > 1 file changed, 40 insertions(+), 17 deletions(-) This patch no longer applies to my tty-next tree, can you please refresh it and resend so that I can apply it? thanks, greg k-h -- 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
