PB1176 broken in -rc1 - Russell King

From: linux@arm.linux.org.uk (Russell King - ARM Linux)
To: linux-arm-kernel@lists.infradead.org
Subject: PB1176 broken in -rc1
Date: Tue, 16 Aug 2011 12:34:46 +0100	[thread overview]
Message-ID: <20110816113446.GA17310@n2100.arm.linux.org.uk> (raw)
In-Reply-To: <20110816101711.GF28656@e102144-lin.cambridge.arm.com>

On Tue, Aug 16, 2011 at 11:17:11AM +0100, Will Deacon wrote:
> Hi Russell,
> 
> On Tue, Aug 16, 2011 at 11:09:06AM +0100, Russell King - ARM Linux wrote:
> > On Tue, Aug 16, 2011 at 10:59:36AM +0100, Will Deacon wrote:
> > > The problem is with earlyprintk because your picking up multiple definitions
> > > of DEBUG_LL_UART_OFFSET as a result of basing your config on the defconfig.
> > > 
> > > Take a look at arch/arm/mach-realview/include/mach/debug-macro.S. I don't
> > > think there's an easy way to fix this because it's used so early, even DT
> > > can't save us.
> > 
> > As I keep saying to people, only use the LL debug during _early_ platform
> > bring-up.  That's what it's there for.
> > 
> > Tying earlyprintk into the LL debug stuff has made the LL debug easier to
> > use, and therefore easier for people to fall into this trap.  That's not
> > the problem of the LL debug stuff, but the problem of its greater exposure.
> > 
> > So, as the LL debug stuff has this rule, so does earlyprintk.  Only use it
> > for early platform bring up and *once* you have a kernel booting through
> > to the proper console, disable it *immediately*.
> > 
> > Anything else will lead you into these pitfalls.
> 
> Yup, I agree about the usage of earlyprintk. It would be good if the user
> could select the platform on which LL debug will work at config time and
> then not have to worry about multiple conflicting definitions of
> DEBUG_LL_UART_OFFSET. That way, you know on which platform earlyprintk
> will work and don't try to use it on any others.

Feel free to send a patch to do that.

One down side of it is that we already have lots of options, so I suspect
it'd get buried and forgotten.  As this is a common problem, it probably
makes sense to put a choice in arch/arm/Kconfig.debug and have individual
LL debug variants depend on that.  We already have:

config DEBUG_DC21285_PORT
        bool "Kernel low-level debugging messages via footbridge serial port"
        depends on DEBUG_LL && FOOTBRIDGE
        help
          Say Y here if you want the debug print routines to direct their
          output to the serial port in the DC21285 (Footbridge). Saying N
          will cause the debug messages to appear on the first 16550
          serial port.

and:

config DEBUG_CLPS711X_UART2
        bool "Kernel low-level debugging messages via UART2"
        depends on DEBUG_LL && ARCH_CLPS711X
        help
          Say Y here if you want the debug print routines to direct their
          output to the second serial port on these devices.  Saying N will
          cause the debug messages to appear on the first serial port.

in there.  So I think the first step is (untested):

diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
index 81cbe40..068337f 100644
--- a/arch/arm/Kconfig.debug
+++ b/arch/arm/Kconfig.debug
@@ -100,9 +100,13 @@ config OC_ETM
 	  buffer driver that will allow you to collect traces of the
 	  kernel code.
 
+choice
+	prompt "Kernel low-level debugging port"
+	depends on DEBUG_LL
+
 config DEBUG_DC21285_PORT
 	bool "Kernel low-level debugging messages via footbridge serial port"
-	depends on DEBUG_LL && FOOTBRIDGE
+	depends on FOOTBRIDGE
 	help
 	  Say Y here if you want the debug print routines to direct their
 	  output to the serial port in the DC21285 (Footbridge). Saying N
@@ -111,12 +115,14 @@ config DEBUG_DC21285_PORT
 
 config DEBUG_CLPS711X_UART2
 	bool "Kernel low-level debugging messages via UART2"
-	depends on DEBUG_LL && ARCH_CLPS711X
+	depends on ARCH_CLPS711X
 	help
 	  Say Y here if you want the debug print routines to direct their
 	  output to the second serial port on these devices.  Saying N will
 	  cause the debug messages to appear on the first serial port.
 
+endchoice
+
 config DEBUG_S3C_UART
 	depends on PLAT_SAMSUNG
 	int "S3C UART to use for low-level debug"
