linux-kernel - Re: [PATCH 2/4] hwmon: Use subdir-ccflags-* to inherit debug flag

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Message-ID: <20210205200859.GA193526@bjorn-Precision-5520>
Date:   Fri, 5 Feb 2021 14:08:59 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Yicong Yang <yangyicong@...ilicon.com>, gregkh@...uxfoundation.org,
        jdelvare@...e.com, giometti@...eenne.com, abbotti@....co.uk,
        hsweeten@...ionengravers.com, kw@...ux.com,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-hwmon@...r.kernel.org, devel@...verdev.osuosl.org,
        linux-kbuild@...r.kernel.org, masahiroy@...nel.org,
        michal.lkml@...kovi.net, prime.zeng@...wei.com,
        linuxarm@...neuler.org
Subject: Re: [PATCH 2/4] hwmon: Use subdir-ccflags-* to inherit debug flag

On Fri, Feb 05, 2021 at 10:28:32AM -0800, Guenter Roeck wrote:
> On Fri, Feb 05, 2021 at 05:44:13PM +0800, Yicong Yang wrote:
> > From: Junhao He <hejunhao2@...ilicon.com>
> > 
> > Use subdir-ccflags-* instead of ccflags-* to inherit the debug
> > settings from Kconfig when traversing subdirectories.
> > 
> > Suggested-by: Bjorn Helgaas <bhelgaas@...gle.com>
> > Signed-off-by: Junhao He <hejunhao2@...ilicon.com>
> > Signed-off-by: Yicong Yang <yangyicong@...ilicon.com>
> 
> What problem does this fix ? Maybe I am missing it, but I don't see
> DEBUG being used in a subdirectory of drivers/hwmon.

It's my fault for raising this question [1].  Yicong fixed a real
problem in drivers/pci, where we are currently using

  ccflags-$(CONFIG_PCI_DEBUG) := -DDEBUG

so CONFIG_PCI_DEBUG=y turns on debug in drivers/pci, but not in the
subdirectories.  That's surprising to users.

So my question was whether we should default to using subdir-ccflags
for -DDEBUG in general, and only use ccflags when we have
subdirectories that have their own debug options, e.g.,

  drivers/i2c/Makefile:ccflags-$(CONFIG_I2C_DEBUG_CORE) := -DDEBUG
  drivers/i2c/algos/Makefile:ccflags-$(CONFIG_I2C_DEBUG_ALGO) := -DDEBUG
  drivers/i2c/busses/Makefile:ccflags-$(CONFIG_I2C_DEBUG_BUS) := -DDEBUG
  drivers/i2c/muxes/Makefile:ccflags-$(CONFIG_I2C_DEBUG_BUS) := -DDEBUG

I mentioned drivers/hwmon along with a few others that have
subdirectories, do not have per-subdirectory debug options, and use
ccflags.  I didn't try to determine whether those subdirectories
currently use -DDEBUG.

In the case of drivers/hwmon, several drivers do use pr_debug(),
and CONFIG_HWMON_DEBUG_CHIP=y turns those on.  But if somebody
were to add pr_debug() to drivers/hwmon/occ/common.c, for example,
CONFIG_HWMON_DEBUG_CHIP=y would *not* turn it on.  That sounds
surprising to me, but if that's what you intend, that's totally fine.

[1] https://lore.kernel.org/r/20210204161048.GA68790@bjorn-Precision-5520

> > ---
> >  drivers/hwmon/Makefile | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/hwmon/Makefile b/drivers/hwmon/Makefile
> > index 09a86c5..1c0c089 100644
> > --- a/drivers/hwmon/Makefile
> > +++ b/drivers/hwmon/Makefile
> > @@ -201,5 +201,5 @@ obj-$(CONFIG_SENSORS_XGENE)	+= xgene-hwmon.o
> >  obj-$(CONFIG_SENSORS_OCC)	+= occ/
> >  obj-$(CONFIG_PMBUS)		+= pmbus/
> >  
> > -ccflags-$(CONFIG_HWMON_DEBUG_CHIP) := -DDEBUG
> > +subdir-ccflags-$(CONFIG_HWMON_DEBUG_CHIP) := -DDEBUG
> >  
> > -- 
> > 2.8.1
> >