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Date:   Fri, 24 Apr 2020 11:13:25 +0100
From:   Daniel Thompson <daniel.thompson@linaro.org>
To:     Sumit Garg <sumit.garg@linaro.org>
Cc:     Douglas Anderson <dianders@chromium.org>,
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Subject: Re: [PATCH v2 0/9] kgdb: Support late serial drivers; enable early
 debug w/ boot consoles
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On Fri, Apr 24, 2020 at 02:02:51PM +0530, Sumit Garg wrote:
> Hi Doug,
> 
> On Wed, 22 Apr 2020 at 02:45, Douglas Anderson <dianders@chromium.org> wrote:
> >
> > This whole pile of patches was motivated by me trying to get kgdb to
> > work properly on a platform where my serial driver ended up being hit
> > by the -EPROBE_DEFER virus (it wasn't practicing social distancing
> > from other drivers).  Specifically my serial driver's parent device
> > depended on a resource that wasn't available when its probe was first
> > called.  It returned -EPROBE_DEFER which meant that when "kgdboc"
> > tried to run its setup the serial driver wasn't there.  Unfortunately
> > "kgdboc" never tried again, so that meant that kgdb was disabled until
> > I manually enalbed it via sysfs.
> >
> > While I could try to figure out how to get around the -EPROBE_DEFER
> > somehow, the above problems could happen to anyone and -EPROBE_DEFER
> > is generally considered something you just have to live with.  In any
> > case the current "kgdboc" setup is a bit of a race waiting to happen.
> > I _think_ I saw during early testing that even adding a msleep() in
> > the typical serial driver's probe() is enough to trigger similar
> > issues.
> >
> > I decided that for the above race the best attitude to get kgdb to
> > register at boot was probably "if you can't beat 'em, join 'em".
> > Thus, "kgdboc" now jumps on the -EPROBE_DEFER bandwagon (now that my
> > driver uses it it's no longer a virus).  It does so a little awkwardly
> > because "kgdboc" hasn't normally had a "struct device" associated with
> > it, but it's really not _that_ ugly to make a platform device and
> > seems less ugly than alternatives.
> >
> > Unfortunately now on my system the debugger is one of the last things
> > to register at boot.  That's OK for debugging problems that show up
> > significantly after boot, but isn't so hot for all the boot problems
> > that I end up debugging.  This motivated me to try to get something
> > working a little earlier.
> >
> > My first attempt was to try to get the existing "ekgdboc" to work
> > earlier.  I tried that for a bit until I realized that it needed to
> > work at the tty layer and I couldn't find any serial drivers that
> > managed to register themselves to the tty layer super early at boot.
> > The only documented use of "ekgdboc" is "ekgdboc=kbd" and that's a bit
> > of a special snowflake.  Trying to get my serial driver and all its
> > dependencies to probe normally and register the tty driver super early
> > at boot seemed like a bad way to go.  In fact, all the complexity
> > needed to do something like this is why the system already has a
> > special concept of a "boot console" that lives only long enough to
> > transition to the normal console.
> >
> > Leveraging the boot console seemed like a good way to go and that's
> > what this series does.  I found that consoles could have a read()
> > function, though I couldn't find anyone who implemented it.  I
> > implemented it for two serial drivers for the devices I had easy
> > access to, making the assumption that for boot consoles that we could
> > assume read() and write() were polling-compatible (seems sane I
> > think).
> >
> > Now anyone who makes a small change to their serial driver can easily
> > enable early kgdb debugging!
> >
> > The devices I had for testing were:
> > - arm32: rk3288-veyron-jerry
> > - arm64: rk3399-gru-kevin
> > - arm64: qcom-sc7180-trogdor (not mainline yet)
> >
> > These are the devices I tested this series on.  I tried to test
> > various combinations of enabling/disabling various options and I
> > hopefully caught the corner cases, but I'd appreciate any extra
> > testing people can do.
> 
> earlycon_kgdboc sounds like a really cool feature. So I gave it a try
> on my arm64 machine (Developerbox) and it works like a charm. So for
> patch 6/9 you can add:
> 
> Tested-by: Sumit Garg <sumit.garg@linaro.org>
> 
> Plus, in order to enable earlycon_kgdboc on Developerbox I had to
> implement the read() function in the early console driver for
> amba-pl011 (see patch [1]). It would be great if you could pick that
> patch [1] too as part of this series.
> 
> [1] https://lkml.org/lkml/2020/4/24/173

I think PL011 support is also useful for getting this feature integrated
into the test suite too!


Daniel.


> 
> -Sumit
> 
> >  Notably I didn't test on x86, but (I think) I
> > didn't touch much there so I shouldn't have broken anything.
> >
> > When testing I found a few problems with actually dropping into the
> > debugger super early on arm and arm64 devices.  Patches in this series
> > should help with this.  For arm I just avoid dropping into the
> > debugger until a little later and for arm64 I actually enable
> > debugging super early.
> >
> > I realize that bits of this series might feel a little hacky, though
> > I've tried to do things in the cleanest way I could without overly
> > interferring with the rest of the kernel.  If you hate the way I
> > solved a problem I would love it if you could provide guidance on how
> > you think I could solve the problem better.
> >
> > This series (and my comments / documentation / commit messages) are
> > now long enough that my eyes glaze over when I try to read it all over
> > to double-check.  I've nontheless tried to double-check it, but I'm
> > pretty sure I did something stupid.  Thank you ahead of time for
> > pointing it out to me so I can fix it in v3.  If somehow I managed to
> > not do anything stupid (really?) then thank you for double-checking me
> > anyway.
> >
> > Changes in v2:
> > - ("kgdb: Disable WARN_CONSOLE_UNLOCKED for all kgdb") new for v2.
> > - ("Revert "kgdboc: disable the console lock when in kgdb"") new for v2.
> > - Assumes we have ("kgdb: Disable WARN_CONSOLE_UNLOCKED for all kgdb")
> > - Fix kgdbts, tty/mips_ejtag_fdc, and usb/early/ehci-dbgp
> >
> > Douglas Anderson (9):
> >   kgdb: Disable WARN_CONSOLE_UNLOCKED for all kgdb
> >   Revert "kgdboc: disable the console lock when in kgdb"
> >   kgdboc: Use a platform device to handle tty drivers showing up late
> >   kgdb: Delay "kgdbwait" to dbg_late_init() by default
> >   arm64: Add call_break_hook() to early_brk64() for early kgdb
> >   kgdboc: Add earlycon_kgdboc to support early kgdb using boot consoles
> >   Documentation: kgdboc: Document new earlycon_kgdboc parameter
> >   serial: qcom_geni_serial: Support earlycon_kgdboc
> >   serial: 8250_early: Support earlycon_kgdboc
> >
> >  .../admin-guide/kernel-parameters.txt         |  20 ++
> >  Documentation/dev-tools/kgdb.rst              |  14 +
> >  arch/arm64/include/asm/debug-monitors.h       |   2 +
> >  arch/arm64/kernel/debug-monitors.c            |   2 +-
> >  arch/arm64/kernel/kgdb.c                      |   5 +
> >  arch/arm64/kernel/traps.c                     |   3 +
> >  arch/x86/kernel/kgdb.c                        |   5 +
> >  drivers/misc/kgdbts.c                         |   2 +-
> >  drivers/tty/mips_ejtag_fdc.c                  |   2 +-
> >  drivers/tty/serial/8250/8250_early.c          |  23 ++
> >  drivers/tty/serial/kgdboc.c                   | 262 ++++++++++++++++--
> >  drivers/tty/serial/qcom_geni_serial.c         |  32 +++
> >  drivers/usb/early/ehci-dbgp.c                 |   2 +-
> >  include/linux/kgdb.h                          |  25 +-
> >  kernel/debug/debug_core.c                     |  48 +++-
> >  15 files changed, 400 insertions(+), 47 deletions(-)
> >
> > --
> > 2.26.1.301.g55bc3eb7cb9-goog
> >
> >
> > _______________________________________________
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> > linux-arm-kernel@lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel