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 v2. If somehow I managed to not do anything stupid (really?) then thank you for double-checking me anyway. Douglas Anderson (7): 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/tty/serial/8250/8250_early.c | 23 ++ drivers/tty/serial/kgdboc.c | 266 ++++++++++++++++-- drivers/tty/serial/qcom_geni_serial.c | 32 +++ include/linux/kgdb.h | 25 +- kernel/debug/debug_core.c | 44 ++- 12 files changed, 401 insertions(+), 40 deletions(-) -- 2.26.0.110.g2183baf09c-goog
