ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Vineet Gupta]

+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).

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].

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

Thx,
-Vineet

[1] http://lists.busybox.net/pipermail/buildroot/2011-July/044505.html
[2] http://lists.busybox.net/pipermail/buildroot/2011-August/044832.html

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Re: ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[John Ogness]

Hi Vineet,

On 2021-01-06, Vineet Gupta <vgupta@synopsys.com> wrote:
> This breaks ARC booting (no output on console).

Could you provide the kernel boot arguments that you use? This series is
partly about addressing users that have used boot arguments that are
technically incorrect (even if had worked). Seeing the boot arguments of
users that are not experiencing problems may help to reveal some of the
unusual console usages until now.

John Ogness

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Re: ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Greg Ungerer]

Hi John,

On 7/1/21 7:02 pm, John Ogness wrote:
> On 2021-01-06, Vineet Gupta <vgupta@synopsys.com> wrote:
>> This breaks ARC booting (no output on console).
> 
> Could you provide the kernel boot arguments that you use? This series is
> partly about addressing users that have used boot arguments that are
> technically incorrect (even if had worked). Seeing the boot arguments of
> users that are not experiencing problems may help to reveal some of the
> unusual console usages until now.

I can show an example for m68knommu which this change breaks too
(with no console output on boot).

All the ColdFire dev board targets (arch/m68k/configs/m5*) have a 
compiled in boot argument which is "root=/dev/mtdblock0". They have no
real mechanism to pass boot arguments from their boot loader, so it is
compiled in.

The default mcf serial driver is the console on these and no 
"console=ttyS0" argument was required in the past.

Regards
Greg



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Re: ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Vineet Gupta]

Hi John,

On 1/7/21 1:02 AM, John Ogness wrote:
> Hi Vineet,
> 
> On 2021-01-06, Vineet Gupta <vgupta@synopsys.com> wrote:
>> This breaks ARC booting (no output on console).
> 
> Could you provide the kernel boot arguments that you use? This series is
> partly about addressing users that have used boot arguments that are
> technically incorrect (even if had worked). Seeing the boot arguments of
> users that are not experiencing problems may help to reveal some of the
> unusual console usages until now.


Kernel command line: earlycon=uart8250,mmio32,0xf0005000,115200n8 
console=ttyS0,115200n8 debug print-fatal-signals=1

And we do have serial driver built-in.

Thx,
-Vineet

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Re: ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Petr Mladek]

On Thu 2021-01-07 08:43:16, Vineet Gupta wrote:
> Hi John,
> 
> On 1/7/21 1:02 AM, John Ogness wrote:
> > Hi Vineet,
> > 
> > On 2021-01-06, Vineet Gupta <vgupta@synopsys.com> wrote:
> > > This breaks ARC booting (no output on console).
> > 
> > Could you provide the kernel boot arguments that you use? This series is
> > partly about addressing users that have used boot arguments that are
> > technically incorrect (even if had worked). Seeing the boot arguments of
> > users that are not experiencing problems may help to reveal some of the
> > unusual console usages until now.
> 
> 
> Kernel command line: earlycon=uart8250,mmio32,0xf0005000,115200n8
> console=ttyS0,115200n8 debug print-fatal-signals=1

This is strange, the problematic patch should use ttynull
only as a fallback. It should not be used when a particular console
is defined on the command line.

The only explanation would be that ttyS0 gets registered too late
and ttynull is added as a fallback in the meantime.

Anyway, I propose the revert the problematic patch for 5.11-rc3,
see
https://lore.kernel.org/lkml/20210107164400.17904-2-pmladek@suse.com/
This mystery is a good reason to avoid bigger changes at this stage.

Best Regards,
Petr

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Re: ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Vineet Gupta]

On 1/7/21 9:04 AM, Petr Mladek wrote:
> On Thu 2021-01-07 08:43:16, Vineet Gupta wrote:
>> Hi John,
>>
>> On 1/7/21 1:02 AM, John Ogness wrote:
>>> Hi Vineet,
>>>
>>> On 2021-01-06, Vineet Gupta <vgupta@synopsys.com> wrote:
>>>> This breaks ARC booting (no output on console).
>>>
>>> Could you provide the kernel boot arguments that you use? This series is
>>> partly about addressing users that have used boot arguments that are
>>> technically incorrect (even if had worked). Seeing the boot arguments of
>>> users that are not experiencing problems may help to reveal some of the
>>> unusual console usages until now.
>>
>>
>> Kernel command line: earlycon=uart8250,mmio32,0xf0005000,115200n8
>> console=ttyS0,115200n8 debug print-fatal-signals=1
> 
> This is strange, the problematic patch should use ttynull
> only as a fallback. It should not be used when a particular console
> is defined on the command line.

What happens in my case is console_on_rootfs() doesn't find /dev/console 
and switching to ttynull. /dev is not present because devtmpfs doesn't 
automount for initramfs.

> The only explanation would be that ttyS0 gets registered too late
> and ttynull is added as a fallback in the meantime.

I don't know if ttyS0 console should have registered already but even if 
it did - the /dev node missing would not have helped ?

> 
> Anyway, I propose the revert the problematic patch for 5.11-rc3,
> see
> https://lore.kernel.org/lkml/20210107164400.17904-2-pmladek@suse.com/
> This mystery is a good reason to avoid bigger changes at this stage.
> 
> Best Regards,
> Petr
> 


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Re: ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Sergey Senozhatsky]

On (21/01/07 09:58), Vineet Gupta wrote:
> On 1/7/21 9:04 AM, Petr Mladek wrote:
> > On Thu 2021-01-07 08:43:16, Vineet Gupta wrote:
> > > Hi John,
> > > 
> > > On 1/7/21 1:02 AM, John Ogness wrote:
> > > > Hi Vineet,
> > > > 
> > > > On 2021-01-06, Vineet Gupta <vgupta@synopsys.com> wrote:
> > > > > This breaks ARC booting (no output on console).
> > > > 
> > > > Could you provide the kernel boot arguments that you use? This series is
> > > > partly about addressing users that have used boot arguments that are
> > > > technically incorrect (even if had worked). Seeing the boot arguments of
> > > > users that are not experiencing problems may help to reveal some of the
> > > > unusual console usages until now.
> > > 
> > > 
> > > Kernel command line: earlycon=uart8250,mmio32,0xf0005000,115200n8
> > > console=ttyS0,115200n8 debug print-fatal-signals=1
> > 
> > This is strange, the problematic patch should use ttynull
> > only as a fallback. It should not be used when a particular console
> > is defined on the command line.
> 
> What happens in my case is console_on_rootfs() doesn't find /dev/console and
> switching to ttynull. /dev is not present because devtmpfs doesn't automount
> for initramfs.

I wonder if we'll move the nulltty fallback logic into printk code [1]
will it fix the problem?

[1] https://lore.kernel.org/lkml/X6x%2FAxD1qanC6evJ@jagdpanzerIV.localdomain/

	-ss

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Re: ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Vineet Gupta]

On 1/7/21 7:48 PM, Sergey Senozhatsky wrote:
> On (21/01/07 09:58), Vineet Gupta wrote:
>> On 1/7/21 9:04 AM, Petr Mladek wrote:
>>> On Thu 2021-01-07 08:43:16, Vineet Gupta wrote:
>>>> Hi John,
>>>>
>>>> On 1/7/21 1:02 AM, John Ogness wrote:
>>>>> Hi Vineet,
>>>>>
>>>>> On 2021-01-06, Vineet Gupta <vgupta@synopsys.com> wrote:
>>>>>> This breaks ARC booting (no output on console).
>>>>>
>>>>> Could you provide the kernel boot arguments that you use? This series is
>>>>> partly about addressing users that have used boot arguments that are
>>>>> technically incorrect (even if had worked). Seeing the boot arguments of
>>>>> users that are not experiencing problems may help to reveal some of the
>>>>> unusual console usages until now.
>>>>
>>>>
>>>> Kernel command line: earlycon=uart8250,mmio32,0xf0005000,115200n8
>>>> console=ttyS0,115200n8 debug print-fatal-signals=1
>>>
>>> This is strange, the problematic patch should use ttynull
>>> only as a fallback. It should not be used when a particular console
>>> is defined on the command line.
>>
>> What happens in my case is console_on_rootfs() doesn't find /dev/console and
>> switching to ttynull. /dev is not present because devtmpfs doesn't automount
>> for initramfs.
> 
> I wonder if we'll move the nulltty fallback logic into printk code [1]
> will it fix the problem?
> 
> [1] https://lore.kernel.org/lkml/X6x%2FAxD1qanC6evJ@jagdpanzerIV.localdomain/

Your reasoning in the post above makes total sense.

I tired the patch: adding register_ttynull_console() call in 
console_device(), removing from console_on_rootfs() band that works too.


| Warning: unable to open an initial console. Fallback to ttynull.
| Warning: Failed to add ttynull console. No stdin, stdout, and stderr 
for the init process!
| Freeing unused kernel memory: 3096K
| This architecture does not have kernel memory protection.
| Run /init as init process
| with arguments:
|    /init
|  with environment:
|    HOME=/
|    TERM=linux
| Starting System logger (syslogd)
| Bringing up loopback device
| Starting inetd
| Mounting Posix Mqueue filesys
| 
CONFIG_INITRAMFS_SOURCE="~/arc/RAMFS/archs/ramfs_2011-GNU-2020-03-glibc-2.32-tiny"
| **********************************************************************
|			Welcome to ARCLinux
| **********************************************************************
| [ARCLinux]#

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Re: ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Petr Mladek]

On Thu 2021-01-07 21:18:20, Vineet Gupta wrote:
> On 1/7/21 7:48 PM, Sergey Senozhatsky wrote:
> > On (21/01/07 09:58), Vineet Gupta wrote:
> > > On 1/7/21 9:04 AM, Petr Mladek wrote:
> > > > On Thu 2021-01-07 08:43:16, Vineet Gupta wrote:
> > > > > Hi John,
> > > > > 
> > > > > On 1/7/21 1:02 AM, John Ogness wrote:
> > > > > > Hi Vineet,
> > > > > > 
> > > > > > On 2021-01-06, Vineet Gupta <vgupta@synopsys.com> wrote:
> > > > > > > This breaks ARC booting (no output on console).
> > > > > > 
> > > > > > Could you provide the kernel boot arguments that you use? This series is
> > > > > > partly about addressing users that have used boot arguments that are
> > > > > > technically incorrect (even if had worked). Seeing the boot arguments of
> > > > > > users that are not experiencing problems may help to reveal some of the
> > > > > > unusual console usages until now.
> > > > > 
> > > > > 
> > > > > Kernel command line: earlycon=uart8250,mmio32,0xf0005000,115200n8
> > > > > console=ttyS0,115200n8 debug print-fatal-signals=1
> > > > 
> > > > This is strange, the problematic patch should use ttynull
> > > > only as a fallback. It should not be used when a particular console
> > > > is defined on the command line.
> > > 
> > > What happens in my case is console_on_rootfs() doesn't find /dev/console and
> > > switching to ttynull. /dev is not present because devtmpfs doesn't automount
> > > for initramfs.

I see. I did not though about a possibility that /dev/console could
not be opened from other reasons.

> > I wonder if we'll move the nulltty fallback logic into printk code [1]
> > will it fix the problem?
> > 
> > [1] https://lore.kernel.org/lkml/X6x%2FAxD1qanC6evJ@jagdpanzerIV.localdomain/
> 
> Your reasoning in the post above makes total sense.
> 
> I tired the patch: adding register_ttynull_console() call in
> console_device(), removing from console_on_rootfs() band that works too.

IMHO, this worked because you removed the change in console_on_rootfs().

I guess that the change in console_device() did not make any
difference. It was likely not called because
filp_open("/dev/console", O_RDWR, 0) failed earlier because
/dev/ did not exists.

Anyway, the proposed change in console_device() has some
more problems that I realized this week:

   + It does not check whether console_drivers really contains
     any console with tty binding.

   + register_ttynull_console() calls
    add_preferred_console(ttynull_console.name, 0, NULL).
    The ttynull console stays preferred even when any better
    console gets registered later. As a result, it would
    stay associated with /dev/console.

The right solution would be to enable ttynull console and
do _not_ modify the list of preferred consoles. And it makes
sense to add the console only there is no console with
tty binding at the moment.

I still have to think whether console_device() is a better or
worse location for adding tyynull as a fallback.

Anyway, it has to wait. The proper solution can't be done easily
with the existing register_console() code. We need to clean
it up first.

Best Regards,
Petr

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Re: ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Peter Korsgaard]

>>>>> "Vineet" == Vineet Gupta <vgupta@synopsys.com> writes:

 > On 1/7/21 9:04 AM, Petr Mladek wrote:
 >> On Thu 2021-01-07 08:43:16, Vineet Gupta wrote:
 >>> Hi John,
 >>> 
 >>> On 1/7/21 1:02 AM, John Ogness wrote:
 >>>> Hi Vineet,
 >>>> 
 >>>> On 2021-01-06, Vineet Gupta <vgupta@synopsys.com> wrote:
 >>>>> This breaks ARC booting (no output on console).
 >>>> 
 >>>> Could you provide the kernel boot arguments that you use? This series is
 >>>> partly about addressing users that have used boot arguments that are
 >>>> technically incorrect (even if had worked). Seeing the boot arguments of
 >>>> users that are not experiencing problems may help to reveal some of the
 >>>> unusual console usages until now.
 >>> 
 >>> 
 >>> Kernel command line: earlycon=uart8250,mmio32,0xf0005000,115200n8
 >>> console=ttyS0,115200n8 debug print-fatal-signals=1
 >> 
 >> This is strange, the problematic patch should use ttynull
 >> only as a fallback. It should not be used when a particular console
 >> is defined on the command line.

 > What happens in my case is console_on_rootfs() doesn't find
 > /dev/console and switching to ttynull. /dev is not present because
 > devtmpfs doesn't automount for initramfs.

But our initramfs/cpio logic ensures that the initramfs has a static
/dev/console device node, so how can that be?

https://git.buildroot.net/buildroot/tree/fs/cpio/cpio.mk#n25

-- 
Bye, Peter Korsgaard

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Re: ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Richard Weinberger]

[CC'in linux-um since there is a similar issue]




On Thu, Jan 7, 2021 at 12:38 AM Vineet Gupta <vgupta@synopsys.com> 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).
>
> 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].
>
> 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
>
> Thx,
> -Vineet
>
> [1] http://lists.busybox.net/pipermail/buildroot/2011-July/044505.html
> [2] http://lists.busybox.net/pipermail/buildroot/2011-August/044832.html



--
Thanks,
//richard

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Re: ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Petr Mladek]

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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Re: [Buildroot] ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Yann E. MORIN]

Vineet, All,

On 2021-01-06 15:36 -0800, Vineet Gupta spake thusly:
> On 11/11/20 5:54 AM, Petr Mladek wrote:
[--SNIP--]
> >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).
> 
> 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 "$@"

I guess you are speaking about the initramfs (cpio) case, right?

We've changed that code last August:

    https://git.buildroot.org/buildroot/commit/fs/cpio/init?id=b9026e83f

I.e. if we can't do the redirection, then we don't redirect anything.
The change was done for people who explicitly pass an empty console= on
their kernel command line.

Now, I haven't looked at nulltty yet, and I have (so far) no idea on how
it works. Thanks for the hint, I'll have a look.

> Buildroot has had this way of handling missing /dev/console since 2011 [1]
> and [2].

See also more archaelogy on that topic, referenced in that commit:
    https://git.buildroot.org/buildroot/commit/fs/cpio/?id=98a6f1fc02e41

> Please advise what needs to be done to unbork boot.

This has been present since the 2020.08 release, and has been backported
to the maintenance branches:
    2020.02.x (LTS) -> f1a83afe2df2a
    2020.05.x       -> 797f9e40224c9

> 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

I won't speak about whether this is a kernel regression or not, not my
call.

Regards,
Yann E. MORIN.

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