[U-Boot] [PATCH 2/2] armv8: make SPL exception vectors optional

[Tom Rini]

On Wed, Jul 25, 2018 at 12:57:01AM +0100, Andre Przywara wrote:

> Even though the exception vector table is a fundamental part of the ARM
> architecture, U-Boot mostly does not make real use of it, except when
> crash dumping. But having it in takes up quite some space, partly due to
> the architectural alignment requirement of 2KB. Since we don't take special
> care of that, the compiler adds a more or less random amount of padding
> space, which increases the image size quite a bit, especially for the SPL.
> 
> On a typical Allwinner build this is around 1.5KB of padding, plus 1KB
> for the vector table (mostly padding space again), then some extra code
> to do the actual handling. This amounts to almost 10% of the maximum image
> size, which is quite a lot for a pure debugging feature.
> 
> Add a Kconfig symbol to allow the exception vector table to be left out
> of the build for the SPL.
> For now this is "default y" for everyone, but specific defconfigs,
> platforms or .config files can opt out here at will, to mitigate the code
> size pressure we see for some SPLs.
> 
> Signed-off-by: Andre Przywara <andre.przywara at arm.com>
> ---
>  arch/arm/cpu/armv8/Kconfig  | 11 +++++++++++
>  arch/arm/cpu/armv8/Makefile |  4 ++++
>  arch/arm/cpu/armv8/start.S  | 19 +++++++++++++++----
>  3 files changed, 30 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm/cpu/armv8/Kconfig b/arch/arm/cpu/armv8/Kconfig
> index 22d2f29548..bd1e759c37 100644
> --- a/arch/arm/cpu/armv8/Kconfig
> +++ b/arch/arm/cpu/armv8/Kconfig
> @@ -1,5 +1,16 @@
>  if ARM64
>  
> +config ARMV8_SPL_EXCEPTION_VECTORS
> +	bool "Install crash dump exception vectors"
> +	depends on SPL
> +	default y
> +	help
> +	  The default exception vector table is only used for the crash
> +	  dump, but still takes quite a lot of space in the image size.
> +
> +	  Say N here if you are running out of code space in the image
> +	  and want to save some space at the cost of less debugging info.
> +
>  config ARMV8_MULTIENTRY
>          bool "Enable multiple CPUs to enter into U-Boot"
>  
> diff --git a/arch/arm/cpu/armv8/Makefile b/arch/arm/cpu/armv8/Makefile
> index d1d4ffecfd..52c8daa049 100644
> --- a/arch/arm/cpu/armv8/Makefile
> +++ b/arch/arm/cpu/armv8/Makefile
> @@ -10,7 +10,11 @@ ifndef CONFIG_$(SPL_TPL_)TIMER
>  obj-$(CONFIG_SYS_ARCH_TIMER) += generic_timer.o
>  endif
>  obj-y	+= cache_v8.o
> +ifdef CONFIG_SPL_BUILD
> +obj-$(CONFIG_ARMV8_SPL_EXCEPTION_VECTORS) += exceptions.o
> +else
>  obj-y	+= exceptions.o
> +endif

I _think_ we can make the code parts of this read better (and make use
of CONFIG_IS_ENABLED() if we add CONFIG_ARMV8_EXCEPTION_VECTORS and
don't allow it to be unselected (ie no text on the bool line).

-- 
Tom
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