On Thu, Nov 19, 2009 at 05:38:55PM +0100, Jiri Kosina wrote: > On Wed, 18 Nov 2009, Jiri Kosina wrote: > > > The patch below has been lost several times (the last submission happened > > probably on [1]). I am currently aware of a real hardware which triggers > > this problem quite reliably, so we should rather have that really fixed. > > > > [1] http://lkml.org/lkml/2009/3/12/379 > > > > > > > > > > From: Ian Jackson <ian.jackson@xxxxxxxxxxxxx> > > > > Do not read IIR in serial8250_start_tx when UART_BUG_TXEN > > > > Reading the IIR clears some oustanding interrupts so it is not safe. > > Instead, simply transmit immediately if the buffer is empty without > > regard to IIR. > > > > Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx> > > Reviewed-by: Markus Armbruster <armbru@xxxxxxxxxx> > > Reviewed-by: Jiri Kosina <jkosina@xxxxxxx> > > > > --- > > drivers/serial/8250.c | 8 +++----- > > 1 files changed, 3 insertions(+), 5 deletions(-) > > > > diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c > > index 737b4c9..807042b 100644 > > --- a/drivers/serial/8250.c > > +++ b/drivers/serial/8250.c > > @@ -1339,14 +1339,12 @@ static void serial8250_start_tx(struct uart_port *port) > > serial_out(up, UART_IER, up->ier); > > > > if (up->bugs & UART_BUG_TXEN) { > > - unsigned char lsr, iir; > > + unsigned char lsr; > > lsr = serial_in(up, UART_LSR); > > up->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS; > > - iir = serial_in(up, UART_IIR) & 0x0f; > > if ((up->port.type == PORT_RM9000) ? > > - (lsr & UART_LSR_THRE && > > - (iir == UART_IIR_NO_INT || iir == UART_IIR_THRI)) : > > - (lsr & UART_LSR_TEMT && iir & UART_IIR_NO_INT)) > > + (lsr & UART_LSR_THRE) : > > + (lsr & UART_LSR_TEMT)) > > transmit_chars(up); > > } > > } > > Does anyone have any comments about this please? > > In a nutshell -- this is needed so that we make the UART_BUG_TXEN really > harmless (which I guess it originally inteded to be, but reading IIR has > some unwanted sideeffects and is in fact not needed). > > We need to have this in to handle properly the cases in which BUG_TXEN is > misdetected, and we can't blacklist such systems as we do for some SoL > hardware (see commit b6adea334c6c (" 8250: fix boot hang with serial > console when using with Serial Over Lan port"). Also there doesn't seem to > be any straightforward way to workaround the misdetection, so this seems > to be proper fix, unbreaking all the possible scenarios. > > Alan, Greg, any comments? At first glance, it looks acceptable to me. It's in my "to-apply" queue that has gotten a bit big due to "real work" and a vacation I took last week. 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
