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SubjectRe: [PATCHv2 2/2] ARM: debug: Move DEBUG_ICEDCC into the DEBUG_LL choice
Hi Russell,

On Thu, Sep 29, 2011 at 10:46:02AM +0100, Russell King - ARM Linux wrote:
> On Thu, Sep 29, 2011 at 10:42:43AM +0100, Will Deacon wrote:
> > Bah, just took this for a spin on my Realview-PBX (Cortex-A9) platform and
> > the board won't boot unless I have a hardware debugger enabled that can
> > service the comms channel. This contradicts the Kconfig text which says:
> >
> > It does include a timeout to ensure that the system does not
> > totally freeze when there is nothing connected to read.
> >
> > so we certainly need to fix something!
>
> One of the problems you'll encounter is that there's places where we
> don't have any timer infrastructure (eg, decompressor) and software
> timing loops won't work (they'll be too slow for the lower-end CPUs
> and too fast for the upper-end CPUs.)

Agreed, software timing loops are the only option and it's impossible to get
them right everywhere. I think the Kconfig help text should be updated so
that the timeout paragraph is removed.

> The best fix is probably to leave that as-is, but introduce a 'default'
> output entry instead - so platforms which don't have any listed will
> get that instead of the ICEDDC stuff.

Ok. Rather than 'default' (which people will probably select in preference
to choosing the correct UART) how about something like below? I can split
this into two patches if you prefer.

Will


----8<----
diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
index 32b1be4..bfdf86b 100644
--- a/arch/arm/Kconfig.debug
+++ b/arch/arm/Kconfig.debug
@@ -81,6 +81,14 @@ choice
prompt "Kernel low-level debugging port"
depends on DEBUG_LL

+ config DEBUG_LL_UART_NONE
+ bool "No low-level debugging UART"
+ help
+ Say Y here if your platform doesn't provide a UART option
+ below. This relies on your platform choosing the right UART
+ definition internally in order for low-level debugging to
+ work.
+
config DEBUG_ICEDCC
bool "Kernel low-level debugging via EmbeddedICE DCC channel"
help
@@ -89,8 +97,8 @@ choice
co-processor 14. This is known to work on the ARM9 style ICE
channel and on the XScale with the PEEDI.

- It does include a timeout to ensure that the system does not
- totally freeze when there is nothing connected to read.
+ Note that this may cause the system to hang during boot if
+ there is nothing to read from the DCC.

config DEBUG_FOOTBRIDGE_COM1
bool "Kernel low-level debugging messages via footbridge 8250 at PCI COM1"


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