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

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