[PATCH v2 0/4] printk: nbcon: deprecate boot_delay in favour of printk_delay

[Andrew Murray]

The boot_delay (BOOT_PRINTK_DELAY) kernel parameter and printk_delay
sysctl are two distinct mechanisms for providing similar functionality
which add a delay prior to each printed printk message.

boot_delay provides a kernel parameter for delaying printk output from
kernel start through to boot (SYSTEM_RUNNING), whereas printk_delay is
configurable only via sysctl and thus is only used post boot.

However, since the introduction of nbcon and the legacy printer thread
for PREEMPT_RT kernels, printk records are now emited to the console
asynchronously to the caller of printk. Thus, any printk delay added by
boot_delay/printk_delay continues to slow down the calling process but
may not have any impact to the rate in which records are emited to the
console, especially for slow consoles.

To address these issues, let's deprecate boot_delay, extend printk_delay
to be useable from kernel start and ensure that delays occur at the point
where console messages are printed rather than queued.

Please note that this patchset results in delays occuring after a message
is printed rather than, as it is now, before.

Signed-off-by: Andrew Murray <amurray@xxxxxxxxxxxxxxxxxxxx>
---
Please see the following related work for additional context:

- https://lore.kernel.org/all/20260503214214.3475670-1-rdunlap@xxxxxxxxxxxxx/
- https://lore.kernel.org/all/20260505-printk_delay-v1-1-5dba51d7f17c@xxxxxxxxxxxxxxxxxxxx/
- https://lore.kernel.org/r/20260601-deprecate_boot_delay-v1-0-c34c187142a6@xxxxxxxxxxxxxxxxxxxx (v1)

---
Changes in v2:
- Rebased onto v7.2-rc1
- Correctly handle negative values for printk_delay_msec (patch 2)
- Add missing newline in pr_warn (patch 2)
- Improved patch descriptions for (patches 2 and 3)
- Use new emitted flag in nbcon_context/nbcon_write_context in place of backlog && wctxt.len checks (patch 3)
- Use unsigned char for unsafe_takeover field instead of bool in nbcon_write_context (patch 3)
- Move printk_delay_msec and printk_delay from printk.h to internal.h (patch 3)
- Revert regression added in v1 to __nbcon_atomic_flush_pending_con (patch 3)
- Move printk_delay later in console_emit_next_record ensuring delay is always after emit (across nbcon/legacy) (patch 3)
- Fixed typo in documentation s/boot_delay/printk_delay/g in printk_delay= section (patch 4)
- Link to v1: https://lore.kernel.org/r/20260601-deprecate_boot_delay-v1-0-c34c187142a6@xxxxxxxxxxxxxxxxxxxx

---
Andrew Murray (4):
      printk: remove BOOT_PRINTK_DELAY config option
      printk: deprecate boot_delay in favour of printk_delay
      printk: nbcon: move printk_delay to console emiting code
      Documentation/kernel-parameters: add/update printk_delay/boot_delay

 Documentation/admin-guide/kernel-parameters.txt | 31 +++++++--
 arch/arm/configs/bcm2835_defconfig              |  1 -
 include/linux/console.h                         |  5 +-
 include/linux/printk.h                          |  1 -
 kernel/printk/internal.h                        |  6 ++
 kernel/printk/nbcon.c                           | 13 ++++
 kernel/printk/printk.c                          | 84 +++++++++++++++----------
 lib/Kconfig.debug                               | 18 ------
 8 files changed, 101 insertions(+), 58 deletions(-)
---
base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
change-id: 20260515-deprecate_boot_delay-72516da3845a

Best regards,
-- 
Andrew Murray <amurray@xxxxxxxxxxxxxxxxxxxx>