On Friday, January 10, 2014 04:13:54 PM Al Stone wrote: > On 01/10/2014 04:11 PM, Rafael J. Wysocki wrote: > > On Friday, January 10, 2014 03:52:15 PM al.stone@xxxxxxxxxx wrote: > >> From: Al Stone <al.stone@xxxxxxxxxx> > >> > >> 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@xxxxxxxxxx> > >> Signed-off-by: Al Stone <al.stone@xxxxxxxxxx> > >> --- > >> 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? > > Hrm. Do you mean something like this: > > config ACPI_REDUCED_HARDWARE_ONLY > bool "Hardware-reduced ACPI support only" > default n > depends on EXPERT > help > ... > > and remove any mention of architecture? Yes. > Or forgo the "depends" entirely? From my perspective, any of the above is sufficient > so I'll defer to your broader view of ACPI. Well, I thought about something like this: config ACPI_REDUCED_HARDWARE_ONLY def_bool n depends on ACPI && EXPERT and then do the above select in arch-level Kconfigs where necessary. Thanks! -- I speak only for myself. Rafael J. Wysocki, Intel Open Source Technology Center. -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
