The recent patch ("kgdboc: Add kgdboc_earlycon 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 "kgdboc_earlycon" 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 "kgdboc_earlycon" works
in combination with "kgdboc" and is backed by boot consoles.
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
---
Changes in v4:
- s/The the/The/
Changes in v3:
- Added example in kgdb.rst
- Removed unneeded sentence in kerenel-parameters doc.
- Renamed earlycon_kgdboc to kgdboc_earlycon.
- Suggest people use kgdboc_earlycon instead of ekgdboc.
Changes in v2: None
.../admin-guide/kernel-parameters.txt | 20 ++++++++++++++++
Documentation/dev-tools/kgdb.rst | 24 +++++++++++++++++++
2 files changed, 44 insertions(+)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 7bc83f3d9bdf..2cbde9ea476d 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1190,6 +1190,11 @@
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. For early debugging
+ via a serial port see kgdboc_earlycon instead.
+
edd= [EDD]
Format: {"off" | "on" | "skip[mbr]"}
@@ -2105,6 +2110,21 @@
kms, kbd format: kms,kbd
kms, kbd and serial format: kms,kbd,<ser_dev>[,baud]
+ kgdboc_earlycon= [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 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. It's OK to leave the value
+ blank and the first boot console that implements
+ read() will be picked.
+
kgdbwait [KGDB] Stop kernel execution and enter the
kernel debugger at the earliest opportunity.
diff --git a/Documentation/dev-tools/kgdb.rst b/Documentation/dev-tools/kgdb.rst
index d38be58f872a..61293f40bc6e 100644
--- a/Documentation/dev-tools/kgdb.rst
+++ b/Documentation/dev-tools/kgdb.rst
@@ -274,6 +274,30 @@ 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: ``kgdboc_earlycon``
+-------------------------------------
+
+If you specify the kernel parameter ``kgdboc_earlycon`` 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 ``kgdboc_earlycon`` 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. Note that names that
+are registered through the boot console layer and the tty layer are not
+the same for the same port.
+
+For instance, on one board to be explicit you might do::
+
+ kgdboc_earlycon=qcom_geni kgdboc=ttyMSM0
+
+If the only boot console on the device was "qcom_geni", you could simplify::
+
+ kgdboc_earlycon kgdboc=ttyMSM0
+
Kernel parameter: ``kgdbwait``
------------------------------
--
2.26.2.645.ge9eca65c58-goog
