Hi Doug, On Wed, 22 Apr 2020 at 02:45, Douglas Anderson <dianders@xxxxxxxxxxxx> 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@xxxxxxxxxx> 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 -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 > > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
