Subject: Relinking zImage when nothing changes

From: [email-masked] (Stephen Boyd)
Date: Fri, 12 Aug 2011 11:12:11 -0700

On 08/12/2011 11:02 AM, Sam Ravnborg wrote:
> On Fri, Aug 12, 2011 at 10:46:24AM -0700, Stephen Boyd wrote:
>> I was hoping with the recent patch e78e8f2 (kernel: prevent unnecessary
>> rebuilding due to config_data.gz, 2011-07-05) compiling ARM linux a
>> second time would amount to no more linking. This doesn't seem to be the
>> case though. Doing a make V=2 I see that lib1funcs.S is shipped and thus
>> we have to recompile it although nothing actually changed. This in turn
>> requires us to relink the compressed vmlinux and then recreate the zImage.
>>
>>   Kernel: arch/arm/boot/Image is ready
>>   SHIPPED arch/arm/boot/compressed/lib1funcs.S - due to missing .cmd file
>>   AS      arch/arm/boot/compressed/lib1funcs.o - due to: arch/arm/boot/compressed/lib1funcs.S
>>   LD      arch/arm/boot/compressed/vmlinux - due to: arch/arm/boot/compressed/lib1funcs.o
>>   OBJCOPY arch/arm/boot/zImage - due to: arch/arm/boot/compressed/vmlinux
>>   Kernel: arch/arm/boot/zImage is ready
>>
>>
>> Is there any way to avoid this? Perhaps the shipped command could become
>> a bit wiser?
> Following patch will likely fix it:
> diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
> index 0c74a6f..80b6b6e 100644
> --- a/arch/arm/boot/compressed/Makefile
> +++ b/arch/arm/boot/compressed/Makefile
> @@ -123,7 +123,7 @@ LDFLAGS_vmlinux += -T
>  # For __aeabi_uidivmod
>  lib1funcs = $(obj)/lib1funcs.o
>
> -$(obj)/lib1funcs.S: $(srctree)/arch/$(SRCARCH)/lib/lib1funcs.S FORCE
> +$(obj)/lib1funcs.S: $(srctree)/arch/$(SRCARCH)/lib/lib1funcs.S
>         $(call cmd,shipped)
>
>  # We need to prevent any GOTOFF relocs being used with references
>
> (cut'n'pasted ...)
>
> The FORCE prerequisite will tell make to always execute the command.
> But we only want to execute the command if:
> 1) $(obj)/lib1funcs.S is missing
> 2) or $(srctree)/arch/$(SRCARCH)/lib/lib1funcs.S is not older then $(obj)/lib1funcs.S 
>
> So dropping FORCE should be a safe thing to do.
>

Hm... that fixes the shipped part. But now we still reassemble lib1funcs.o.

  Kernel: arch/arm/boot/Image is ready
  AS      arch/arm/boot/compressed/lib1funcs.o - due to lib1funcs.o not in $(targets)
  LD      arch/arm/boot/compressed/vmlinux - due to: arch/arm/boot/compressed/lib1funcs.o
  OBJCOPY arch/arm/boot/zImage - due to: arch/arm/boot/compressed/vmlinux
  Kernel: arch/arm/boot/zImage is ready

Is it correct to add lib1funcs.o to targets?

diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
index 0c74a6f..64889e6 100644
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -91,7 +91,7 @@ suffix_$(CONFIG_KERNEL_LZMA) = lzma
 
 targets       := vmlinux vmlinux.lds \
                 piggy.$(suffix_y) piggy.$(suffix_y).o \
-                font.o font.c head.o misc.o $(OBJS)
+                font.o font.c head.o misc.o lib1funcs.o $(OBJS)
 
 # Make sure files are removed during clean
 extra-y       += piggy.gzip piggy.lzo piggy.lzma lib1funcs.S
@@ -123,7 +123,7 @@ LDFLAGS_vmlinux += -T
 # For __aeabi_uidivmod
 lib1funcs = $(obj)/lib1funcs.o
 
-$(obj)/lib1funcs.S: $(srctree)/arch/$(SRCARCH)/lib/lib1funcs.S FORCE
+$(obj)/lib1funcs.S: $(srctree)/arch/$(SRCARCH)/lib/lib1funcs.S
        $(call cmd,shipped)
 
 # We need to prevent any GOTOFF relocs being used with references

-- 
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