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[209.85.222.51]) by smtp.gmail.com with ESMTPSA id w2sm4622329vso.19.2020.04.28.14.39.58 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 28 Apr 2020 14:39:58 -0700 (PDT) Received: by mail-ua1-f51.google.com with SMTP id i5so153820uaq.1 for ; Tue, 28 Apr 2020 14:39:58 -0700 (PDT) X-Received: by 2002:a67:c40c:: with SMTP id c12mr23285523vsk.106.1588109589393; Tue, 28 Apr 2020 14:33:09 -0700 (PDT) MIME-Version: 1.0 References: <20200421211447.193860-1-dianders@chromium.org> <20200421141234.v2.7.I7d5eb42c6180c831d47aef1af44d0b8be3fac559@changeid> <20200427164619.kw4mihmoxmxzjk66@holly.lan> In-Reply-To: <20200427164619.kw4mihmoxmxzjk66@holly.lan> From: Doug Anderson Date: Tue, 28 Apr 2020 14:32:57 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2 7/9] Documentation: kgdboc: Document new earlycon_kgdboc parameter To: Daniel Thompson Cc: Jason Wessel , Greg Kroah-Hartman , kgdb-bugreport@lists.sourceforge.net, Ingo Molnar , "H. Peter Anvin" , bp@alien8.de, linux-serial@vger.kernel.org, Andy Gross , Thomas Gleixner , Frank Rowand , Bjorn Andersson , Jiri Slaby , Catalin Marinas , Jonathan Corbet , Will Deacon , Andrew Morton , Borislav Petkov , Juergen Gross , Mauro Carvalho Chehab , Pawan Gupta , linux-doc@vger.kernel.org, LKML Content-Type: text/plain; charset="UTF-8" Sender: linux-serial-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-serial@vger.kernel.org Hi, On Mon, Apr 27, 2020 at 9:46 AM Daniel Thompson wrote: > > 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 > > --- > > > > 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. OK. 'git format-patch', sed, and 'git am' for the win. > > + > > 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. Done. > > 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). Done.