Docs: arm64: booting: clarify boot requirements
There are a few points in the arm64 booting document which are unclear
(such as the initial state of secondary CPUs), and/or have not been
documented (PSCI is a supported mechanism for booting secondary CPUs).
This patch amends the arm64 boot document to better express the
(existing) requirements, and to describe PSCI as a supported booting
mechanism.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Martin <dave.martin@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Fu Wei <tekkamanninja@gmail.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
diff --git a/Documentation/arm64/booting.txt b/Documentation/arm64/booting.txt
index 98df4a0..a9691cc 100644
--- a/Documentation/arm64/booting.txt
+++ b/Documentation/arm64/booting.txt
@@ -115,9 +115,10 @@
External caches (if present) must be configured and disabled.
- Architected timers
- CNTFRQ must be programmed with the timer frequency.
- If entering the kernel at EL1, CNTHCTL_EL2 must have EL1PCTEN (bit 0)
- set where available.
+ CNTFRQ must be programmed with the timer frequency and CNTVOFF must
+ be programmed with a consistent value on all CPUs. If entering the
+ kernel at EL1, CNTHCTL_EL2 must have EL1PCTEN (bit 0) set where
+ available.
- Coherency
All CPUs to be booted by the kernel must be part of the same coherency
@@ -130,30 +131,46 @@
the kernel image will be entered must be initialised by software at a
higher exception level to prevent execution in an UNKNOWN state.
+The requirements described above for CPU mode, caches, MMUs, architected
+timers, coherency and system registers apply to all CPUs. All CPUs must
+enter the kernel in the same exception level.
+
The boot loader is expected to enter the kernel on each CPU in the
following manner:
- The primary CPU must jump directly to the first instruction of the
kernel image. The device tree blob passed by this CPU must contain
- for each CPU node:
-
- 1. An 'enable-method' property. Currently, the only supported value
- for this field is the string "spin-table".
-
- 2. A 'cpu-release-addr' property identifying a 64-bit,
- zero-initialised memory location.
+ an 'enable-method' property for each cpu node. The supported
+ enable-methods are described below.
It is expected that the bootloader will generate these device tree
properties and insert them into the blob prior to kernel entry.
-- Any secondary CPUs must spin outside of the kernel in a reserved area
- of memory (communicated to the kernel by a /memreserve/ region in the
+- CPUs with a "spin-table" enable-method must have a 'cpu-release-addr'
+ property in their cpu node. This property identifies a
+ naturally-aligned 64-bit zero-initalised memory location.
+
+ These CPUs should spin outside of the kernel in a reserved area of
+ memory (communicated to the kernel by a /memreserve/ region in the
device tree) polling their cpu-release-addr location, which must be
contained in the reserved region. A wfe instruction may be inserted
to reduce the overhead of the busy-loop and a sev will be issued by
the primary CPU. When a read of the location pointed to by the
- cpu-release-addr returns a non-zero value, the CPU must jump directly
- to this value.
+ cpu-release-addr returns a non-zero value, the CPU must jump to this
+ value. The value will be written as a single 64-bit little-endian
+ value, so CPUs must convert the read value to their native endianness
+ before jumping to it.
+
+- CPUs with a "psci" enable method should remain outside of
+ the kernel (i.e. outside of the regions of memory described to the
+ kernel in the memory node, or in a reserved area of memory described
+ to the kernel by a /memreserve/ region in the device tree). The
+ kernel will issue CPU_ON calls as described in ARM document number ARM
+ DEN 0022A ("Power State Coordination Interface System Software on ARM
+ processors") to bring CPUs into the kernel.
+
+ The device tree should contain a 'psci' node, as described in
+ Documentation/devicetree/bindings/arm/psci.txt.
- Secondary CPU general-purpose register settings
x0 = 0 (reserved for future use)
