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    SubjectRe: [PATCH v2 7/9] Documentation: kgdboc: Document new earlycon_kgdboc parameter
    On Tue, Apr 21, 2020 at 02:14:45PM -0700, Douglas Anderson wrote:
    > 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@chromium.org>
    > ---
    >
    > Changes in v2: None
    >
    > .../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 f2a93c8679e8..588625ec2993 100644
    > --- a/Documentation/admin-guide/kernel-parameters.txt
    > +++ b/Documentation/admin-guide/kernel-parameters.txt
    > @@ -1132,6 +1132,22 @@
    > 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.

    There's no need for the "If only one boot console with a read()
    funcuiton is enabled" here,

    Seeing this in alphabetic order in this patch it also crosses my mind
    that kgdboc_earlycon might be a better name so that is sorts closer
    to the other kgdb options. This is a kgdboc feature that uses earlycon
    not an earlycon feature that uses kgdboc.


    > +
    > earlyprintk= [X86,SH,ARM,M68k,S390]
    > earlyprintk=vga
    > earlyprintk=sclp
    > @@ -1190,6 +1206,10 @@
    > 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.
    > +

    I wonder if pragmatic advice is more useful:

    For early debugging via a serial port see earlycon_kgdboc instead.

    > 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.
    > +

    I think we need an example here. The example in the patch header for
    the previous patch was useful (at least for me).


    Daniel.

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