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    SubjectRe: ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)
    On Wed 2021-01-06 15:36:36, Vineet Gupta wrote:
    > +CC Buildroot folks
    >
    > Hi Petr,
    >
    > On 11/11/20 5:54 AM, Petr Mladek wrote:
    > > stdin, stdout, and stderr standard I/O stream are created for the init
    > > process. They are not available when there is no console registered
    > > for /dev/console. It might lead to a crash when the init process
    > > tries to use them, see the commit 48021f98130880dd742 ("printk: handle
    > > blank console arguments passed in.").
    > >
    > > Normally, ttySX and ttyX consoles are used as a fallback when no consoles
    > > are defined via the command line, device tree, or SPCR. But there
    > > will be no console registered when an invalid console name is configured
    > > or when the configured consoles do not exist on the system.
    > >
    > > Users even try to avoid the console intentionally, for example,
    > > by using console="" or console=null. It is used on production
    > > systems where the serial port or terminal are not visible to
    > > users. Pushing messages to these consoles would just unnecessary
    > > slowdown the system.
    > >
    > > Make sure that stdin, stdout, stderr, and /dev/console are always
    > > available by a fallback to the existing ttynull driver. It has
    > > been implemented for exactly this purpose but it was used only
    > > when explicitly configured.
    > >
    > > Signed-off-by: Petr Mladek <pmladek@suse.com>
    >
    > > --- a/init/main.c
    > > +++ b/init/main.c
    > > @@ -1470,8 +1470,14 @@ void __init console_on_rootfs(void)
    > > struct file *file = filp_open("/dev/console", O_RDWR, 0);
    > > if (IS_ERR(file)) {
    > > - pr_err("Warning: unable to open an initial console.\n");
    > > - return;
    > > + pr_err("Warning: unable to open an initial console. Fallback to ttynull.\n");
    > > + register_ttynull_console();
    > > +
    > > + file = filp_open("/dev/console", O_RDWR, 0);
    > > + if (IS_ERR(file)) {
    > > + pr_err("Warning: Failed to add ttynull console. No stdin, stdout, and stderr for the init process!\n");
    > > + return;
    > > + }
    >
    >
    > This breaks ARC booting (no output on console).

    This is likely the same problem as with kunit and um kernels.
    It is being discussed at
    https://lore.kernel.org/linux-kselftest/X%2FSRA1P8t+ONZFKb@alley/#t

    We have several workarounds. I am still squashing my head about the
    right solution. The console registration code is like a vasps' nest.
    It is always a pain when we touch it.

    I hope that I will send a patchset for review later today.
    In the worst case, we will revert the patch in the mainline.

    > Our Buildroot based setup has dynamic /dev where /dev/console doesn't exist
    > statically and there's a primoridla /init shell script which does following
    >
    > /bin/mount -t devtmpfs devtmpfs /dev
    > exec 0</dev/console
    > exec 1>/dev/console
    > exec 2>/dev/console
    > exec /sbin/init "$@"
    >
    > Buildroot has had this way of handling missing /dev/console since 2011 [1]
    > and [2].

    Good to know.

    > Please advise what needs to be done to unbork boot. Otherwise this seems
    > like a kernel change which breaks user-space and needs to be backed-out (or
    > perhaps conditionalize on CONFIG_NULL_TTY. I'm surprised it hasn't been
    > reported by any other embedded folks

    Two workarounds can be fount at
    https://lore.kernel.org/linux-kselftest/X%2FSYhBZyudfnKY1u@alley/
    https://lore.kernel.org/linux-kselftest/X%2FW2sl7RMvfaV4Ru@alley/

    But I still see them as only a partial solutiuon. I still another sources
    of potential problems.

    Best Regards,
    Petr

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