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Subject[PATCH v2 7/9] Documentation: kgdboc: Document new earlycon_kgdboc parameter
The recent patch ("kgdboc: Add earlycon_kgdboc to support early kgdb
using boot consoles") adds a new kernel command line parameter.
Document it.

Note that the patch adding the feature does some comparing/contrasting
of "earlycon_kgdboc" vs. the existing "ekgdboc". See that patch for
more details, but briefly "ekgdboc" can be used _instead_ of "kgdboc"
and just makes "kgdboc" do its normal initialization early (only works
if your tty driver is already ready). The new "earlycon_kgdboc" works
in combination with "kgdboc" and is backed by boot consoles.

Signed-off-by: Douglas Anderson <>

Changes in v2: None

.../admin-guide/kernel-parameters.txt | 20 +++++++++++++++++++
Documentation/dev-tools/kgdb.rst | 14 +++++++++++++
2 files changed, 34 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f2a93c8679e8..588625ec2993 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1132,6 +1132,22 @@
address must be provided, and the serial port must
already be setup and configured.

+ earlycon_kgdboc= [KGDB,HW]
+ If the boot console provides the ability to read
+ characters and can work in polling mode, you can use
+ this parameter to tell kgdb to use it as a backend
+ until the normal console is registered. Intended to
+ be used together with the kgdboc parameter which
+ specifies the normal console to transition to.
+ The the name of the early console should be specified
+ as the value of this parameter. Note that the name of
+ the early console might be different than the tty
+ name passed to kgdboc. If only one boot console with
+ a read() function is enabled it's OK to leave the
+ value blank and the first boot console that implements
+ read() will be picked.
earlyprintk= [X86,SH,ARM,M68k,S390]
@@ -1190,6 +1206,10 @@
This is designed to be used in conjunction with
the boot argument: earlyprintk=vga

+ This parameter works in place of the kgdboc parameter
+ but can only be used if the backing tty is available
+ very early in the boot process.
edd= [EDD]
Format: {"off" | "on" | "skip[mbr]"}

diff --git a/Documentation/dev-tools/kgdb.rst b/Documentation/dev-tools/kgdb.rst
index d38be58f872a..c0b321403d9a 100644
--- a/Documentation/dev-tools/kgdb.rst
+++ b/Documentation/dev-tools/kgdb.rst
@@ -274,6 +274,20 @@ don't like this are to hack gdb to send the :kbd:`SysRq-G` for you as well as
on the initial connect, or to use a debugger proxy that allows an
unmodified gdb to do the debugging.

+Kernel parameter: ``earlycon_kgdboc``
+If you specify the kernel parameter ``earlycon_kgdboc`` and your serial
+driver registers a boot console that supports polling (doesn't need
+interrupts and implements a nonblocking read() function) kgdb will attempt
+to work using the boot console until it can transition to the regular
+tty driver specified by the ``kgdboc`` parameter.
+Normally there is only one boot console (especially that implements the
+read() function) so just adding ``earlycon_kgdboc`` on its own is
+sufficient to make this work. If you have more than one boot console you
+can add the boot console's name to differentiate.
Kernel parameter: ``kgdbwait``

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