On Thu, 20 Sep 2007 18:28:49 +0100 (BST) "Maciej W. Rozycki" <macro@xxxxxxxxxxxxxx> wrote: > Move the hadover message to after the boot console has been released to > avoid bad interactions between it and the real console. > > Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxxxxxx> > --- > The 69331af79cf29e26d1231152a172a1a10c2df511 commit of May 8th added a > "console handover: ..." message to register_console() that is output > during the short period when both the boot and the newly-registered > console are registered. > > This is presumably fine for boot consoles implemented entirely by Linux > as they are fully controlled. But it may produce problems when the boot > console is actually implemented as a call to the firmware which may not be > quite happy about how the OS driver for the piece of hardware involved > controls it. > > I hit this problem with the DECstation. Depending on the configuration > the fimrware uses a graphics adapter or a predefined serial port for > console output -- which device actually that is cannot be reliably > determined by Linux, though an approximation may be possible. Now if the > firmware uses the serial port and Linux is asked to use the same serial > port for the real console, then this printk() hangs forever in the > firmware. The driver used is drivers/serial/zs.c. > > The reason is by the time the ->write() call is issued for the boot > console as a result of this printk(), the zs.c driver has been initialised > and because at the moment the serial port has not been opened, the serial > transmitter is disabled. The firmware polls for the transmit buffer empty > condition, but does not enable the module, presumably under the assumption > it will not be called once an OS driver has taken control of the device > (the register containing the enable bit is write-only anyway, so it would > be hard to restore the previous value). This causes a hang, because once > a single character is put into the transmit buffer it will not become > empty until the transmitter has been enabled. > > The serial console as implemented by zs.c handles the case correctly, by > enabling the transmitter, outputting what should be output, waiting for > the transmit shift register to drain and restoring the state of the > transmitter enable (which is held in a shadow variable). > > I feel a bit uneasy about keeping serial transmitters enabled for lines > that have not been opened; I gather others may agree as for example while > not explicitly mentioned, I believe it is implied by what is said in > Documentation/serial/driver referring to the ->shutdown() call: "Disable > the port, [...]" -- with the transmitter enabled a port can hardly be > considered fully disabled. Below is a change which makes the problem > disappear for me, but I suppose there was a deliberate reason for placing > the printk() where it is now and nowhere else. > > Any suggestions will be appreciated. > > Maciej > > patch-mips-2.6.23-rc5-20070904-printk-handover-0 > diff -up --recursive --new-file linux-mips-2.6.23-rc5-20070904.macro/kernel/printk.c linux-mips-2.6.23-rc5-20070904/kernel/printk.c > --- linux-mips-2.6.23-rc5-20070904.macro/kernel/printk.c 2007-09-04 04:56:21.000000000 +0000 > +++ linux-mips-2.6.23-rc5-20070904/kernel/printk.c 2007-09-19 21:10:16.000000000 +0000 > @@ -1014,11 +1014,11 @@ void register_console(struct console *co > return; > > if (bootconsole && (console->flags & CON_CONSDEV)) { > + unregister_console(bootconsole); > + console->flags &= ~CON_PRINTBUFFER; > printk(KERN_INFO "console handover: boot [%s%d] -> real [%s%d]\n", > bootconsole->name, bootconsole->index, > console->name, console->index); > - unregister_console(bootconsole); > - console->flags &= ~CON_PRINTBUFFER; > } else { > printk(KERN_INFO "console [%s%d] enabled\n", > console->name, console->index); It would be useful to have some basic information like: Which kernel version was this found in? Which kernel version last worked? - 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