[PATCH 5/7] Documentation: kgdboc: Document new earlycon_kgdboc parameter

[Douglas Anderson]

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 <dianders@xxxxxxxxxxxx>
---

 .../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 c07815d230bc..37dc2db13743 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1124,6 +1124,22 @@
 			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]
 			earlyprintk=vga
 			earlyprintk=sclp
@@ -1182,6 +1198,10 @@
 			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``
 ------------------------------
 
-- 
2.26.0.110.g2183baf09c-goog