Re: [PATCH v2 0/9] kgdb: Support late serial drivers; enable early debug w/ boot consoles

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Apr 21, 2020 at 02:14:38PM -0700, Douglas Anderson 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.  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(-)

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>