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SubjectRe: RFC 2/2] printk: Restore and document obsolete ways to disable console output
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On 10/22/20 4:42 AM, Petr Mladek wrote:
> The commit 48021f98130880dd7 ("printk: handle blank console arguments
> passed in.") prevented crash caused by empty console= parameter value.
>
> Unfortunately, this value is widely used on Chromebooks to disable
> the console output. The above commit caused performance regression
> because the messages were pushed on slow console even though nobody
> was watching it.
>
> "mute_console" kernel parameter has been introduced as a proper and
> official was to disable console output. It avoids the performance
> problem by suppressing all kernel messages. Also avoids the crash
> by registering the default console.
>
> Make console="" behave the same as "mute_console" to avoid
> the performance regression on existing Chromebooks.
>
> Do the same also for console=null which seem to be another widely
> suggested and non-official way to disable the console output.
>
> Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> ---
> Documentation/admin-guide/kernel-parameters.txt | 5 +++++
> kernel/printk/printk.c | 9 ++++++++-
> 2 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 52b9e7f5468d..14472f674a89 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -670,11 +670,16 @@
> hvc<n> Use the hypervisor console device <n>. This is for
> both Xen and PowerPC hypervisors.
>
> + null
> + "" Obsolete way to disable console output. Please,
> + use "mute_console" instead.
> +
> If the device connected to the port is not a TTY but a braille
> device, prepend "brl," before the device type, for instance
> console=brl,ttyS0
> For now, only VisioBraille is supported.
>
> +

stray whitespace change

> console_msg_format=
> [KNL] Change console messages format
> default
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 63fb96630767..08c50d8ba110 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2208,8 +2208,15 @@ static int __init console_setup(char *str)
> char *s, *options, *brl_options = NULL;
> int idx;
>
> - if (str[0] == 0)
> + /*
> + * console="" or console=null have been suggested as a way to
> + * disable console output. It worked just by chance and was not
> + * reliable. It has been obsoleted by "mute_console" parameter.
> + */
> + if (str[0] == 0 || strcmp(str, "null") == 0) {
> + mute_console = true;
> return 1;
> + }
>
> if (_braille_console_setup(&str, &brl_options))
> return 1;
>

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