[PATCH v6 1/6] ACPI: introduce CONFIG_ACPI_REDUCED_HARDWARE_ONLY to enforce this ACPI mode

[Rafael J. Wysocki]

On Friday, January 10, 2014 03:52:15 PM al.stone at linaro.org wrote:
> From: Al Stone <al.stone at linaro.org>
> 
> Hardware reduced mode, despite the name, exists primarily to allow
> newer platforms to use a much simpler form of ACPI that does not
> require supporting the legacy of previous versions of the specification.
> This mode was first introduced in the ACPI 5.0 specification, but because
> it is so much simpler and reduces the size of the object code needed to
> support ACPI, it is likely to be used more often in the near future.
> 
> To enable the hardware reduced mode of ACPI on some platforms (such as
> ARM), we need to modify the kernel code and set ACPI_REDUCED_HARDWARE
> to TRUE in the ACPICA source.  For ARM/ARM64, hardware reduced ACPI
> should be the only mode used; legacy mode would require modifications
> to SoCs in order to provide several x86-specific hardware features (e.g.,
> an NMI and SMI support).
> 
> We set ACPI_REDUCED_HARDWARE to TRUE in the ACPICA source by introducing
> a kernel config item to enable/disable ACPI_REDUCED_HARDWARE.  We can then
> change the kernel config instead of having to modify the kernel source
> directly to enable the reduced hardware mode of ACPI.
> 
> Lv Zheng suggested that this configuration item does not belong in ACPICA,
> the upstream source for much of the ACPI internals, but rather to the
> Linux kernel itself.  Hence, we introduce this flag so that we can make
> ACPI_REDUCED_HARDWARE configurable.  For the details of the discussion,
> please refer to: http://www.spinics.net/lists/linux-acpi/msg46369.html
> 
> Even though support for X86 in hardware reduced mode is possible, it
> is NOT enabled.  Extensive effort has gone into the Linux kernel so that
> there is a single kernel image than can run on all x86 hardware; the kernel
> changes run-time behavior to adapt to the hardware being used.  This is not
> currently possible with the existing ACPICA infrastructure but only presents
> a problem on achitectures supporting both hardware-reduced and legacy modes
> of ACPI -- i.e., on x86 only.
> 
> The problem with the current ACPICA code base is that if one builds legacy
> ACPI (a proper superset of hardware-reduced), the kernel can run in hardware-
> reduced with the proper ACPI tables, but there is still ACPICA code that could
> be executed even though it is not allowed by the specification.  If one builds
> a hardware-reduced only ACPI, the kernel cannot run with ACPI tables that are
> for legacy mode.  To ensure compliance with ACPI, one must therefore build
> two separate kernels.  Once this problem has been properly fixed, we can then
> enable x86 hardware-reduced mode and use a single kernel.
> 
> Signed-off-by: Hanjun Guo <hanjun.guo at linaro.org>
> Signed-off-by: Al Stone <al.stone at linaro.org>
> ---
>  drivers/acpi/Kconfig            | 12 ++++++++++++
>  include/acpi/platform/aclinux.h |  6 ++++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/drivers/acpi/Kconfig b/drivers/acpi/Kconfig
> index 4770de5..75dd38a 100644
> --- a/drivers/acpi/Kconfig
> +++ b/drivers/acpi/Kconfig
> @@ -343,6 +343,18 @@ config ACPI_BGRT
>  	  data from the firmware boot splash. It will appear under
>  	  /sys/firmware/acpi/bgrt/ .
>  
> +config ACPI_REDUCED_HARDWARE_ONLY
> +	bool "Hardware-reduced ACPI support only"
> +	depends on !X86 && !IA64 || EXPERT
> +	help
> +	This config item changes the way the ACPI code is built.  When this
> +	option is selected, the kernel will use a specialized version of
> +	ACPICA that ONLY supports the ACPI "reduced hardware" mode.  The
> +	resulting kernel will be smaller but it will also be restricted to
> +	running in ACPI reduced hardware mode ONLY.
> +
> +	If you are unsure what to do, do not enable this option.

I'm still not exactly convinced this is the best way to do that.

In particular, I wonder if it would be viable to make it disabled by default
and then make the interested architectures do

	select ACPI_REDUCED_HARDWARE_ONLY if ACPI

in their top-level Kconfig files?

> +
>  source "drivers/acpi/apei/Kconfig"
>  
>  config ACPI_EXTLOG
> diff --git a/include/acpi/platform/aclinux.h b/include/acpi/platform/aclinux.h
> index 28f4f4d..7d71f08 100644
> --- a/include/acpi/platform/aclinux.h
> +++ b/include/acpi/platform/aclinux.h
> @@ -52,6 +52,12 @@
>  
>  #ifdef __KERNEL__
>  
> +/* Compile for reduced hardware mode only with this kernel config */
> +
> +#ifdef CONFIG_ACPI_REDUCED_HARDWARE_ONLY
> +#define ACPI_REDUCED_HARDWARE 1
> +#endif
> +
>  #include <linux/string.h>
>  #include <linux/kernel.h>
>  #include <linux/ctype.h>
> 

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.