Re: [PATCH] kernel/printk.c: Concerns about the console handover

Andrew Morton
2007-09-21T08:09:10Z

On Thu, 20 Sep 2007 18:28:49 +0100 (BST) "Maciej W. Rozycki" <macro@linux-mips.org> 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@linux-mips.org>
> ---
> 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?