Documentation: kgdboc: Document new kgdboc_earlycon parameter

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@chromium.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20200507130644.v4.9.I7d5eb42c6180c831d47aef1af44d0b8be3fac559@changeid
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 7bc83f3..2cbde9e 100644 (file)
--- 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 d38be58..61293f4 100644 (file)
--- 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``
 ------------------------------