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

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

[Douglas Anderson]

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(-)

-- 
2.26.1.301.g55bc3eb7cb9-goog

[PATCH v2 8/9] serial: qcom_geni_serial: Support earlycon_kgdboc

[Douglas Anderson]

Implement the read() function in the early console driver.  With
recent kgdb patches this allows you to use kgdb to debug fairly early
into the system boot.

We only bother implementing this if polling is enabled since kgdb
can't be enabled without that.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

Changes in v2: None

 drivers/tty/serial/qcom_geni_serial.c | 32 +++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
index 6119090ce045..4563d152b39e 100644
--- a/drivers/tty/serial/qcom_geni_serial.c
+++ b/drivers/tty/serial/qcom_geni_serial.c
@@ -1090,6 +1090,36 @@ static void qcom_geni_serial_earlycon_write(struct console *con,
 	__qcom_geni_serial_console_write(&dev->port, s, n);
 }
 
+#ifdef CONFIG_CONSOLE_POLL
+static int qcom_geni_serial_earlycon_read(struct console *con,
+					  char *s, unsigned int n)
+{
+	struct earlycon_device *dev = con->data;
+	struct uart_port *uport = &dev->port;
+	int num_read = 0;
+	int ch;
+
+	while (num_read < n) {
+		ch = qcom_geni_serial_get_char(uport);
+		if (ch == NO_POLL_CHAR)
+			break;
+		s[num_read++] = ch;
+	}
+
+	return num_read;
+}
+
+static void __init qcom_geni_serial_enable_early_read(struct geni_se *se,
+						      struct console *con)
+{
+	geni_se_setup_s_cmd(se, UART_START_READ, 0);
+	con->read = qcom_geni_serial_earlycon_read;
+}
+#else
+static inline void qcom_geni_serial_enable_early_read(struct geni_se *se,
+						      struct console *con) { ; }
+#endif
+
 static int __init qcom_geni_serial_earlycon_setup(struct earlycon_device *dev,
 								const char *opt)
 {
@@ -1136,6 +1166,8 @@ static int __init qcom_geni_serial_earlycon_setup(struct earlycon_device *dev,
 
 	dev->con->write = qcom_geni_serial_earlycon_write;
 	dev->con->setup = NULL;
+	qcom_geni_serial_enable_early_read(&se, dev->con);
+
 	return 0;
 }
 OF_EARLYCON_DECLARE(qcom_geni, "qcom,geni-debug-uart",
-- 
2.26.1.301.g55bc3eb7cb9-goog

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

[Greg KH]

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>

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

[Sumit Garg]

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

-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@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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

[Daniel Thompson]

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
> >
> >
> > _______________________________________________
> > linux-arm-kernel mailing list
> > linux-arm-kernel@lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

Re: [PATCH v2 8/9] serial: qcom_geni_serial: Support earlycon_kgdboc

[Daniel Thompson]

On Tue, Apr 21, 2020 at 02:14:46PM -0700, Douglas Anderson wrote:
> Implement the read() function in the early console driver.  With
> recent kgdb patches this allows you to use kgdb to debug fairly early
> into the system boot.
> 
> We only bother implementing this if polling is enabled since kgdb
> can't be enabled without that.
> 
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
> 
> Changes in v2: None
> 
>  drivers/tty/serial/qcom_geni_serial.c | 32 +++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
> index 6119090ce045..4563d152b39e 100644
> --- a/drivers/tty/serial/qcom_geni_serial.c
> +++ b/drivers/tty/serial/qcom_geni_serial.c
> @@ -1090,6 +1090,36 @@ static void qcom_geni_serial_earlycon_write(struct console *con,
>  	__qcom_geni_serial_console_write(&dev->port, s, n);
>  }
>  
> +#ifdef CONFIG_CONSOLE_POLL
> +static int qcom_geni_serial_earlycon_read(struct console *con,
> +					  char *s, unsigned int n)
> +{
> +	struct earlycon_device *dev = con->data;
> +	struct uart_port *uport = &dev->port;
> +	int num_read = 0;
> +	int ch;
> +
> +	while (num_read < n) {
> +		ch = qcom_geni_serial_get_char(uport);
> +		if (ch == NO_POLL_CHAR)
> +			break;
> +		s[num_read++] = ch;
> +	}
> +
> +	return num_read;
> +}
> +
> +static void __init qcom_geni_serial_enable_early_read(struct geni_se *se,
> +						      struct console *con)
> +{
> +	geni_se_setup_s_cmd(se, UART_START_READ, 0);
> +	con->read = qcom_geni_serial_earlycon_read;
> +}
> +#else
> +static inline void qcom_geni_serial_enable_early_read(struct geni_se *se,
> +						      struct console *con) { ; }

This is pure nitpicking but since I was passing... why the ; ?


Daniel.