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From: Douglas Anderson <>
Cc:,,,,,,,,,,,,,, Douglas Anderson <>,
	Andrew Morton <>,
	Juergen Gross <>,
	Mauro Carvalho Chehab <>,
	Pawan Gupta <>,,
Subject: [PATCH v3 08/11] Documentation: kgdboc: Document new kgdboc_earlycon parameter
Date: Tue, 28 Apr 2020 14:13:48 -0700	[thread overview]
Message-ID: <20200428141218.v3.8.I7d5eb42c6180c831d47aef1af44d0b8be3fac559@changeid> (raw)
In-Reply-To: <>

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 <>
Reviewed-by: Greg Kroah-Hartman <>

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..3b5ae06a98aa 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 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``

  parent reply	other threads:[~2020-04-28 21:14 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-28 21:13 [PATCH v3 00/11] kgdb: Support late serial drivers; enable early debug w/ boot consoles Douglas Anderson
2020-04-28 21:13 ` [PATCH v3 01/11] kgdb: Disable WARN_CONSOLE_UNLOCKED for all kgdb Douglas Anderson
2020-04-28 21:13 ` [PATCH v3 02/11] Revert "kgdboc: disable the console lock when in kgdb" Douglas Anderson
2020-04-28 21:13 ` [PATCH v3 03/11] kgdboc: Use a platform device to handle tty drivers showing up late Douglas Anderson
2020-04-28 21:13 ` [PATCH v3 04/11] kgdb: Delay "kgdbwait" to dbg_late_init() by default Douglas Anderson
2020-04-30 15:49   ` Daniel Thompson
2020-04-30 16:35     ` Doug Anderson
2020-05-04 14:43       ` [Kgdb-bugreport] " Daniel Thompson
2020-04-28 21:13 ` [PATCH v3 05/11] arm64: Add call_break_hook() to early_brk64() for early kgdb Douglas Anderson
2020-05-11 14:59   ` Will Deacon
2020-05-11 22:45     ` Doug Anderson
2020-05-12  7:35       ` Will Deacon
2020-05-12 15:27         ` Doug Anderson
2020-05-13  6:17           ` Will Deacon
2020-05-13 23:08             ` Doug Anderson
2020-04-28 21:13 ` [PATCH v3 06/11] kgdb: Prevent infinite recursive entries to the debugger Douglas Anderson
2020-04-30 16:52   ` Daniel Thompson
2020-04-28 21:13 ` [PATCH v3 07/11] kgdboc: Add kgdboc_earlycon to support early kgdb using boot consoles Douglas Anderson
2020-05-04 14:13   ` Daniel Thompson
2020-04-28 21:13 ` Douglas Anderson [this message]
2020-05-04 14:40   ` [PATCH v3 08/11] Documentation: kgdboc: Document new kgdboc_earlycon parameter Daniel Thompson
2020-04-28 21:13 ` [PATCH v3 09/11] serial: qcom_geni_serial: Support kgdboc_earlycon Douglas Anderson
2020-04-28 21:13 ` [PATCH v3 10/11] serial: 8250_early: " Douglas Anderson
2020-04-28 21:13 ` [PATCH v3 11/11] serial: amba-pl011: " Douglas Anderson

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