Commit f71fc3bc authored by Douglas Anderson's avatar Douglas Anderson Committed by Daniel Thompson
Browse files

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: default avatarDouglas Anderson <dianders@chromium.org>
Reviewed-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: default avatarDaniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20200507130644.v4.9.I7d5eb42c6180c831d47aef1af44d0b8be3fac559@changeid


Signed-off-by: default avatarDaniel Thompson <daniel.thompson@linaro.org>
parent b1350132

Documentation/admin-guide/kernel-parameters.txt

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Original line number Diff line number Diff line
@@ -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.

Documentation/dev-tools/kgdb.rst

+24 −0
Original line number Diff line number Diff line
@@ -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``

------------------------------

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