[PATCH] Kernel debugging: omap: print warning if CONFIG_DEBUG_LL is enabled

[PATCH] Kernel debugging: omap: print warning if CONFIG_DEBUG_LL is enabled

[H. Nikolaus Schaller]

commit d2b310b0234c ("ARM: debug: Use generic 8250 debug_ll for omap2 and omap3/4/5 common uarts")
commit fc23beb8a577 ("ARM: debug: Use generic 8250 debug_ll for omap3/4/5")

switched to generic 8250 debug_ll code which seems to be incompatible
with at least OMAP5 boards (OMAP5EVM, Pyra) if CONFIG_DEBUG_LL is
still enabled in some legacy defconfig. Since this is very hard to
relate to these patches and difficult to identify, let's have the
compiler emit a warning.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
---
 arch/arm/include/debug/omap2plus.S | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm/include/debug/omap2plus.S b/arch/arm/include/debug/omap2plus.S
index 192a7583999c..3e7ea21cde2a 100644
--- a/arch/arm/include/debug/omap2plus.S
+++ b/arch/arm/include/debug/omap2plus.S
@@ -12,6 +12,10 @@
 
 #include <linux/serial_reg.h>
 
+#if defined(CONFIG_DEBUG_LL)
+#warning Please disable CONFIG_DEBUG_LL and enable CONFIG_SERIAL_EARLYCON or OMAP5 devices will not boot
+#endif
+
 /* External port on Zoom2/3 */
 #define ZOOM_UART_BASE		0x10000000
 #define ZOOM_UART_VIRT		0xfa400000
-- 
2.12.2

[PATCH] Kernel debugging: omap: print warning if CONFIG_DEBUG_LL is enabled

[Tony Lindgren]

* H. Nikolaus Schaller <hns@goldelico.com> [171108 21:32]:
> commit d2b310b0234c ("ARM: debug: Use generic 8250 debug_ll for omap2 and omap3/4/5 common uarts")
> commit fc23beb8a577 ("ARM: debug: Use generic 8250 debug_ll for omap3/4/5")
> 
> switched to generic 8250 debug_ll code which seems to be incompatible
> with at least OMAP5 boards (OMAP5EVM, Pyra) if CONFIG_DEBUG_LL is
> still enabled in some legacy defconfig. Since this is very hard to
> relate to these patches and difficult to identify, let's have the
> compiler emit a warning.

Hmm the issue is the existing values in .config as Kconfig.debug
does default XXX if DEBUG_XXX, right?

I think we have this issue in general if you enable DEBUG_LL for
a SoC, then enable it for another SoC using the same .config?

Regards,

Tony

[PATCH] Kernel debugging: omap: print warning if CONFIG_DEBUG_LL is enabled

[Russell King - ARM Linux]

On Wed, Nov 08, 2017 at 02:27:55PM -0800, Tony Lindgren wrote:
> * H. Nikolaus Schaller <hns@goldelico.com> [171108 21:32]:
> > commit d2b310b0234c ("ARM: debug: Use generic 8250 debug_ll for omap2 and omap3/4/5 common uarts")
> > commit fc23beb8a577 ("ARM: debug: Use generic 8250 debug_ll for omap3/4/5")
> > 
> > switched to generic 8250 debug_ll code which seems to be incompatible
> > with at least OMAP5 boards (OMAP5EVM, Pyra) if CONFIG_DEBUG_LL is
> > still enabled in some legacy defconfig. Since this is very hard to
> > relate to these patches and difficult to identify, let's have the
> > compiler emit a warning.
> 
> Hmm the issue is the existing values in .config as Kconfig.debug
> does default XXX if DEBUG_XXX, right?
> 
> I think we have this issue in general if you enable DEBUG_LL for
> a SoC, then enable it for another SoC using the same .config?

As I keep saying, DEBUG_LL is the low level debugging code, which is
there to support people trying to debug the early bringup.  It can
only be configured for use with one particular port.  If that doesn't
match the SoC, then, if you make use of the DEBUG_LL code, it will
fail to boot.

Unfortunately, if you use early_printk rather than earlycon, as
early_printk uses the DEBUG_LL bits, it means that your early_printk
kernel is tied to the selected platform.

All well known issues.  early_printk should not be used in a multi-
platform kernel for this very reason.

We don't need a compiler warning there, we probably need better help
text against DEBUG_LL and against EARLY_PRINTK.

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[PATCH] Kernel debugging: omap: print warning if CONFIG_DEBUG_LL is enabled

[Russell King - ARM Linux]

On Wed, Nov 08, 2017 at 10:36:04PM +0000, Russell King - ARM Linux wrote:
> We don't need a compiler warning there, we probably need better help
> text against DEBUG_LL and against EARLY_PRINTK.

Actually, this is already clearly stated against DEBUG_LL:

          Note that selecting this option will limit the kernel to a single
          UART definition, as specified below. Attempting to boot the kernel
          image on a different platform *will not work*, so this option should
          not be enabled for kernels that are intended to be portable.

and since EARLY_PRINTK depends on DEBUG_LL... I guess people don't
read help texts anymore.

-- 
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[PATCH] Kernel debugging: omap: print warning if CONFIG_DEBUG_LL is enabled

[H. Nikolaus Schaller]

Hi Russel,

> Am 08.11.2017 um 23:38 schrieb Russell King - ARM Linux <linux@armlinux.org.uk>:
> 
> On Wed, Nov 08, 2017 at 10:36:04PM +0000, Russell King - ARM Linux wrote:
>> We don't need a compiler warning there, we probably need better help
>> text against DEBUG_LL and against EARLY_PRINTK.
> 
> Actually, this is already clearly stated against DEBUG_LL:
> 
>          Note that selecting this option will limit the kernel to a single
>          UART definition, as specified below. Attempting to boot the kernel
>          image on a different platform *will not work*, so this option should
>          not be enabled for kernels that are intended to be portable.
> 
> and since EARLY_PRINTK depends on DEBUG_LL... I guess people don't
> read help texts anymore.

Yes, we read, but we face a situation where it doesn't help that it is a
well known problem *for you* or anyone else and that it is described in the help.

We simply had a private defconfig for years and someone enabled DEBUG_LL some
years ago and it worked.

Then we upgrade the kernel for every -rc and it continued to work well. Then,
recently when doing just one update the compiled kernel failed to boot. Nobody
did change and look at the defconfig and its help. We just merged the latest
patches from linus/master.

Do you expect anybody to remember after 3 years of *not* changing DEBUG_LL what
was written in a help message? Or to understand that DEBUG_LL has anything to
do with the mysteriously appearing boot problem? Which can't easily be debugged
since there are no messages despite playing with EARLY_PRINTK/EARLYCON?

It took me several hours of git bisect to find out the problematic commits
and needed help by the author to relate it to DEBUG_LL in the defconfig.

So we are coming from the other end with much less knowledge than you have.
Linux has grown to be a beast that only a handful of people fully understand.
And this patch is to help those poor production kernel maintainers like me
who don't...

A technically different solution would be to automatically disable DEBUG_LL
if it is not compatible with some other setting.

BTW it appears that there was similar code elsewhere long time ago:

http://elixir.free-electrons.com/linux/v3.1.10/source/arch/arm/plat-mxc/include/mach/debug-macro.S

BR and thanks,
Nikolaus Schaller

[PATCH] Kernel debugging: omap: print warning if CONFIG_DEBUG_LL is enabled

[Russell King - ARM Linux]

On Thu, Nov 09, 2017 at 06:44:49AM +0100, H. Nikolaus Schaller wrote:
> Hi Russel,

Nikolus,

> > Am 08.11.2017 um 23:38 schrieb Russell King - ARM Linux <linux@armlinux.org.uk>:
> > 
> > On Wed, Nov 08, 2017 at 10:36:04PM +0000, Russell King - ARM Linux wrote:
> >> We don't need a compiler warning there, we probably need better help
> >> text against DEBUG_LL and against EARLY_PRINTK.
> > 
> > Actually, this is already clearly stated against DEBUG_LL:
> > 
> >          Note that selecting this option will limit the kernel to a single
> >          UART definition, as specified below. Attempting to boot the kernel
> >          image on a different platform *will not work*, so this option should
> >          not be enabled for kernels that are intended to be portable.
> > 
> > and since EARLY_PRINTK depends on DEBUG_LL... I guess people don't
> > read help texts anymore.
> 
> Yes, we read, but we face a situation where it doesn't help that it is a
> well known problem *for you* or anyone else and that it is described in
> the help.

Personally, I'd like early_printk not to be re-using this, but others
disagree.  It's all about knowledge and compromise.

What we don't want is more warnings in the kernel - it's already
hard enough for people to spot the "bad" warnings that they need to
fix to have a working system (such as wrong printf specifiers) that
(a) they're not going to spot this #warning and (b) it's just going
to be more noise for those who do try to spot the warnings.

> Do you expect anybody to remember after 3 years of *not* changing DEBUG_LL
> what was written in a help message? Or to understand that DEBUG_LL has
> anything to do with the mysteriously appearing boot problem? Which can't
> easily be debugged since there are no messages despite playing with
> EARLY_PRINTK/EARLYCON?

Do you expect people to read the build log of their kernel and spot
the additional warning?  You might be on the ball enough to do that,
but not everyone does, or is.  Not everyone logs the output of the
kernel build so they can review it later.  I suspect that many just
set the build going in a terminal, walk away and come back sometime
later when they think it's done.  (This view is based upon the number
of warnings that I've seen crop up that are for things that are real
bugs that others have introduced - had the warnings been spotted, it
would've triggered a "oh, that's not right" moment.)

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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[PATCH] Kernel debugging: omap: print warning if CONFIG_DEBUG_LL is enabled

[H. Nikolaus Schaller]

Hi Russel,

> Am 09.11.2017 um 17:58 schrieb Russell King - ARM Linux <linux@armlinux.org.uk>:
> 
> On Thu, Nov 09, 2017 at 06:44:49AM +0100, H. Nikolaus Schaller wrote:
>> Hi Russel,
> 
> Nikolus,
> 
>>> Am 08.11.2017 um 23:38 schrieb Russell King - ARM Linux <linux@armlinux.org.uk>:
>>> 
>>> On Wed, Nov 08, 2017 at 10:36:04PM +0000, Russell King - ARM Linux wrote:
>>>> We don't need a compiler warning there, we probably need better help
>>>> text against DEBUG_LL and against EARLY_PRINTK.
>>> 
>>> Actually, this is already clearly stated against DEBUG_LL:
>>> 
>>>         Note that selecting this option will limit the kernel to a single
>>>         UART definition, as specified below. Attempting to boot the kernel
>>>         image on a different platform *will not work*, so this option should
>>>         not be enabled for kernels that are intended to be portable.
>>> 
>>> and since EARLY_PRINTK depends on DEBUG_LL... I guess people don't
>>> read help texts anymore.
>> 
>> Yes, we read, but we face a situation where it doesn't help that it is a
>> well known problem *for you* or anyone else and that it is described in
>> the help.
> 
> Personally, I'd like early_printk not to be re-using this, but others
> disagree.  It's all about knowledge and compromise.

Sure, we have to make a lot of compromises...

> 
> What we don't want is more warnings in the kernel - it's already
> hard enough for people to spot the "bad" warnings that they need to
> fix to have a working system (such as wrong printf specifiers) that
> (a) they're not going to spot this #warning and (b) it's just going
> to be more noise for those who do try to spot the warnings.

> 
>> Do you expect anybody to remember after 3 years of *not* changing DEBUG_LL
>> what was written in a help message? Or to understand that DEBUG_LL has
>> anything to do with the mysteriously appearing boot problem? Which can't
>> easily be debugged since there are no messages despite playing with
>> EARLY_PRINTK/EARLYCON?
> 
> Do you expect people to read the build log of their kernel and spot
> the additional warning?

I am doing it and I rarely see warnings by the compiler. Maybe sometimes
during the -rc phase but not in stable.

>  You might be on the ball enough to do that,
> but not everyone does, or is.  Not everyone logs the output of the
> kernel build so they can review it later.  I suspect that many just
> set the build going in a terminal, walk away and come back sometime
> later when they think it's done.

In this case they would have a good trigger and reason to look into the
build log: the kernel they just built isn't booting and doesn't tell
anything. Then I guess for many of us the next logical step would be to
look trough the build log (but not through a defconfig that wasn't touched
for ages).

Well, I have to admit that if someone doesn't try a clean build, it
will not see it again if the initial log wasn't captured somewhere...

So it should more be an #error than a #warning. Then people must notice.
But I don't know if there are situations where this would be too strict.

And in this special case someone may even have the idea to enable DEBUG_LL
to get better error reporting - but find it is already enabled. Not knowing
that this is the reason of the problem. This is what IMHO makes this case
very special.

Or the other option would be to automatically disable DEBUG_LL if it
conflicts. Then, there is no need for a warning that could be ignored.

>  (This view is based upon the number
> of warnings that I've seen crop up that are for things that are real
> bugs that others have introduced - had the warnings been spotted, it
> would've triggered a "oh, that's not right" moment.)

BR and thanks,
Nikolaus

[PATCH] Kernel debugging: omap: print warning if CONFIG_DEBUG_LL is enabled

[Russell King - ARM Linux]

Hi Nikkylos,

On Thu, Nov 09, 2017 at 06:16:29PM +0100, H. Nikolaus Schaller wrote:
> Hi Russel,
> 
> > Am 09.11.2017 um 17:58 schrieb Russell King - ARM Linux <linux@armlinux.org.uk>:
> > 
> > On Thu, Nov 09, 2017 at 06:44:49AM +0100, H. Nikolaus Schaller wrote:
> >> Hi Russel,
> > 
> > Nikolus,
> > 
> >>> Am 08.11.2017 um 23:38 schrieb Russell King - ARM Linux <linux@armlinux.org.uk>:
> >>> 
> >>> On Wed, Nov 08, 2017 at 10:36:04PM +0000, Russell King - ARM Linux wrote:
> >>>> We don't need a compiler warning there, we probably need better help
> >>>> text against DEBUG_LL and against EARLY_PRINTK.
> >>> 
> >>> Actually, this is already clearly stated against DEBUG_LL:
> >>> 
> >>>         Note that selecting this option will limit the kernel to a single
> >>>         UART definition, as specified below. Attempting to boot the kernel
> >>>         image on a different platform *will not work*, so this option should
> >>>         not be enabled for kernels that are intended to be portable.
> >>> 
> >>> and since EARLY_PRINTK depends on DEBUG_LL... I guess people don't
> >>> read help texts anymore.
> >> 
> >> Yes, we read, but we face a situation where it doesn't help that it is a
> >> well known problem *for you* or anyone else and that it is described in
> >> the help.
> > 
> > Personally, I'd like early_printk not to be re-using this, but others
> > disagree.  It's all about knowledge and compromise.
> 
> Sure, we have to make a lot of compromises...
> 
> > 
> > What we don't want is more warnings in the kernel - it's already
> > hard enough for people to spot the "bad" warnings that they need to
> > fix to have a working system (such as wrong printf specifiers) that
> > (a) they're not going to spot this #warning and (b) it's just going
> > to be more noise for those who do try to spot the warnings.
> 
> > 
> >> Do you expect anybody to remember after 3 years of *not* changing DEBUG_LL
> >> what was written in a help message? Or to understand that DEBUG_LL has
> >> anything to do with the mysteriously appearing boot problem? Which can't
> >> easily be debugged since there are no messages despite playing with
> >> EARLY_PRINTK/EARLYCON?
> > 
> > Do you expect people to read the build log of their kernel and spot
> > the additional warning?
> 
> I am doing it and I rarely see warnings by the compiler. Maybe sometimes
> during the -rc phase but not in stable.

The warnings you see are compiler specific, and I hardly ever see builds
that are fully clean.

> >  You might be on the ball enough to do that,
> > but not everyone does, or is.  Not everyone logs the output of the
> > kernel build so they can review it later.  I suspect that many just
> > set the build going in a terminal, walk away and come back sometime
> > later when they think it's done.
> 
> In this case they would have a good trigger and reason to look into the
> build log: the kernel they just built isn't booting and doesn't tell
> anything. Then I guess for many of us the next logical step would be to
> look trough the build log (but not through a defconfig that wasn't touched
> for ages).
> 
> Well, I have to admit that if someone doesn't try a clean build, it
> will not see it again if the initial log wasn't captured somewhere...
> 
> So it should more be an #error than a #warning. Then people must notice.
> But I don't know if there are situations where this would be too strict.

There are - it's perfectly valid to have your kernel, but with DEBUG_LL
enabled and configured correctly so it outputs to a real UART on your
platform.  As I've explained already, DEBUG_LL exists as a way of last
resort to solve the "it doesn't boot" problem, because it gives a
way for the early assembler to be instrumented and debugged before
anything else is up, through the kernel boot sequence and into the C
code.

That is its primary purpose.  Adding a #error or #warning is going to
turn people who most need the infrastructure to solve the "it silently
doesn't boot" away.

Unfortunately, DEBUG_LL is one of those things that is platform specific,
it has to be to guarantee output, so situations such as missing DT can
still produce output.  Unfortunately, it got used for early_printk
because it was easier to add early_printk rather than telling people
to add a printascii() call in kernel/printk/printk.c (which is what I
used to tell people, and what I _still_ do today.)

DEBUG_LL has always been intended for this purpose, and it's under the
"debugging" menus, because it's really not meant to be enabled for
production.  If you have folk who have enabled DEBUG_LL in your
configurations, and then left it enabled beyond this level of debugging,
I have to question the competence of that.

DEBUG_LL may be an extreme example of a config option that can land
your kernel in a non-bootable situation, but there are plenty of other
configuration options that will severely degrade the performance of
your kernel.  Should we go around adding #warning's for those as well?

I know that the kernel configuration is hard - and it's hard because
it's large, but it is worth reviewing the options that you have in
your config from time to time, especially the debug options, and
question whether the debug options that are enabled are appropriate
for the phase of development.

So, if you're doing new development, you should have all the options
that add extra checks enabled (such as lockdep, hang checks, etc).
If you're moving towards the end of the development phase, and you
care about performance, then you need to start turning these options
off, because many of them will slow the kernel down.

The configuration of debug options is not something that should be
static.

I suspect that you probably have a lot of debug options enabled in your
kernel that affect it's performance... maybe you're due to review them
as a whole?

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up
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[PATCH] Kernel debugging: omap: print warning if CONFIG_DEBUG_LL is enabled

[H. Nikolaus Schaller]

Hi Russell,

> Am 09.11.2017 um 18:35 schrieb Russell King - ARM Linux <linux@armlinux.org.uk>:
> 
> Hi Nikkylos,
> 
> On Thu, Nov 09, 2017 at 06:16:29PM +0100, H. Nikolaus Schaller wrote:
>> Hi Russel,
>> 
>>> Am 09.11.2017 um 17:58 schrieb Russell King - ARM Linux <linux@armlinux.org.uk>:
>>> 
>>> On Thu, Nov 09, 2017 at 06:44:49AM +0100, H. Nikolaus Schaller wrote:
>>>> Hi Russel,
>>> 
>>> Nikolus,
>>> 
>>>>> Am 08.11.2017 um 23:38 schrieb Russell King - ARM Linux <linux@armlinux.org.uk>:
>>>>> 
>>>>> On Wed, Nov 08, 2017 at 10:36:04PM +0000, Russell King - ARM Linux wrote:
>>>>>> We don't need a compiler warning there, we probably need better help
>>>>>> text against DEBUG_LL and against EARLY_PRINTK.
>>>>> 
>>>>> Actually, this is already clearly stated against DEBUG_LL:
>>>>> 
>>>>>        Note that selecting this option will limit the kernel to a single
>>>>>        UART definition, as specified below. Attempting to boot the kernel
>>>>>        image on a different platform *will not work*, so this option should
>>>>>        not be enabled for kernels that are intended to be portable.
>>>>> 
>>>>> and since EARLY_PRINTK depends on DEBUG_LL... I guess people don't
>>>>> read help texts anymore.
>>>> 
>>>> Yes, we read, but we face a situation where it doesn't help that it is a
>>>> well known problem *for you* or anyone else and that it is described in
>>>> the help.
>>> 
>>> Personally, I'd like early_printk not to be re-using this, but others
>>> disagree.  It's all about knowledge and compromise.
>> 
>> Sure, we have to make a lot of compromises...
>> 
>>> 
>>> What we don't want is more warnings in the kernel - it's already
>>> hard enough for people to spot the "bad" warnings that they need to
>>> fix to have a working system (such as wrong printf specifiers) that
>>> (a) they're not going to spot this #warning and (b) it's just going
>>> to be more noise for those who do try to spot the warnings.
>> 
>>> 
>>>> Do you expect anybody to remember after 3 years of *not* changing DEBUG_LL
>>>> what was written in a help message? Or to understand that DEBUG_LL has
>>>> anything to do with the mysteriously appearing boot problem? Which can't
>>>> easily be debugged since there are no messages despite playing with
>>>> EARLY_PRINTK/EARLYCON?
>>> 
>>> Do you expect people to read the build log of their kernel and spot
>>> the additional warning?
>> 
>> I am doing it and I rarely see warnings by the compiler. Maybe sometimes
>> during the -rc phase but not in stable.
> 
> The warnings you see are compiler specific, and I hardly ever see builds
> that are fully clean.

I can just contribute my experience that mine are usually.

> 
>>> You might be on the ball enough to do that,
>>> but not everyone does, or is.  Not everyone logs the output of the
>>> kernel build so they can review it later.  I suspect that many just
>>> set the build going in a terminal, walk away and come back sometime
>>> later when they think it's done.
>> 
>> In this case they would have a good trigger and reason to look into the
>> build log: the kernel they just built isn't booting and doesn't tell
>> anything. Then I guess for many of us the next logical step would be to
>> look trough the build log (but not through a defconfig that wasn't touched
>> for ages).
>> 
>> Well, I have to admit that if someone doesn't try a clean build, it
>> will not see it again if the initial log wasn't captured somewhere...
>> 
>> So it should more be an #error than a #warning. Then people must notice.
>> But I don't know if there are situations where this would be too strict.
> 
> There are - it's perfectly valid to have your kernel, but with DEBUG_LL
> enabled and configured correctly so it outputs to a real UART on your
> platform.  As I've explained already, DEBUG_LL exists as a way of last
> resort to solve the "it doesn't boot" problem, because it gives a
> way for the early assembler to be instrumented and debugged before
> anything else is up, through the kernel boot sequence and into the C
> code.

Indeed - except that DEBUG_LL can be the reason for the "it doesn't boot"
which I assume nobody expects. t would be like placing a voltmeter somewhere
and you have power outage.

> 
> That is its primary purpose.  Adding a #error or #warning is going to
> turn people who most need the infrastructure to solve the "it silently
> doesn't boot" away.

This confirms my assumption that a #warning seems more appropriate than
an #error. You can ignore it if you know what you are doing.

> 
> Unfortunately, DEBUG_LL is one of those things that is platform specific,
> it has to be to guarantee output, so situations such as missing DT can
> still produce output.  Unfortunately, it got used for early_printk
> because it was easier to add early_printk rather than telling people
> to add a printascii() call in kernel/printk/printk.c (which is what I
> used to tell people, and what I _still_ do today.)

Yes, I have used printascii() in early days of board bringup (especially
with broken u-boot setup)...

> 
> DEBUG_LL has always been intended for this purpose, and it's under the
> "debugging" menus, because it's really not meant to be enabled for
> production.  If you have folk who have enabled DEBUG_LL in your
> configurations, and then left it enabled beyond this level of debugging,
> I have to question the competence of that.

It is not a matter of competence but experience. Simply assume that someone
with only a little experience did enable it (or even copied from some
nowadays unknown source) some years ago for some good reason (in the
view of the team experience back then) and nobody questioned this decision
afterwards.

And suddenly the system fails completely.

It is the same mental principle how severe bugs (and security issues) stay
for years and are suddenly found to the surprise of everybody.

> DEBUG_LL may be an extreme example of a config option that can land
> your kernel in a non-bootable situation, but there are plenty of other
> configuration options that will severely degrade the performance of
> your kernel.  Should we go around adding #warning's for those as well?

No. If just performance is going down, you can still boot and see messages
and debug other (e.g. driver) issues.

And have all debugging tools around. But you can't play around to debug
a non-booting kernel (except through JTAG).

> 
> I know that the kernel configuration is hard - and it's hard because
> it's large, but it is worth reviewing the options that you have in
> your config from time to time, especially the debug options,

We did that from time to time - but I admit it is not done in a systematic
and well documented way. Something to think about and maybe automate...

sed "s/\(.*DEBUG.*\)=y/# \1 is not set/g" <debug_defconfig >defconfig

> and
> question whether the debug options that are enabled are appropriate
> for the phase of development.
> 
> So, if you're doing new development, you should have all the options
> that add extra checks enabled (such as lockdep, hang checks, etc).
> If you're moving towards the end of the development phase, and you
> care about performance, then you need to start turning these options
> off, because many of them will slow the kernel down.
> 
> The configuration of debug options is not something that should be
> static.
> 
> I suspect that you probably have a lot of debug options enabled in your
> kernel that affect it's performance... maybe you're due to review them
> as a whole?

It would not solve the problem. Just assume we have two configs - one with
debug disabled and one with debug enabled. Now we come to 4.14-rc1 where
the code change happened. We will not immediately notice, but as soon as we
build with the debug defconfig (for debugging something else), we can't
even boot.

So instead of fixing an important and urgent issue, we have to make the
debug kernel boot again...

While this might hint at a config problem, we still do not know which one.
And we look for and find DEBUG_LL enabled which we think is (still) right.

And it wasn't completely wrong before the patches for OMAP were integrated.

While my proposed patch would immediately tell - without runtime performance
influence. And in the non-debug config nobody would even see the #warning
because DEBUG_LL is disabled.

But it is up to you to accept or reject this idea - we for our team have
solved it for us. We simply want to save others from making the same
bad experience again. If this is not welcome, it has to be accepted.

BR and thanks,
Nikolaus