Add two API to save SCB context and restore it, typically
used in suspend to RAM use case.
The scb_context_t and the backup/restore functions are designed to only
handle SCB registers that are:
- Mutable: Their values can be changed by software.
- Configurable: They control system behavior or features.
- Stateful: Their values represent a specific configuration that an
application might want to preserve and restore.
Register excluded from backup/restore are:
1. CPUID (CPUID Base Register)
Motivation for Exclusion: This is a read-only identification register.
2. ICSR (Interrupt Control and State Register)
Motivation for Exclusion (from restoration): While its current value
can be read, directly restoring a saved ICSR value is highly
dangerous and generally unsafe in an RTOS context.
Contains Read-Only Status Bits: A significant portion of ICSR
consists of read-only bits (VECTACTIVE, VECTPENDING, ISRPREEMPT,
TSRUNPEND). These bits reflect the current state of the exception
system (e.g., which exception is active, which are pending) and are
managed dynamically by the CPU and the RTOS.
Forcing a previous state onto these bits would corrupt the live
system's interrupt handling.
Contains Write-Only Set/Clear Bits: Some bits are write-only to set
or clear a pending interrupt (PENDSVSET, PENDSVCLR, SYSTICKSET,
SYSTICKCLR). If these bits were set in the saved context, restoring
them might immediately trigger an interrupt or change its pending state
unexpectedly, outside the RTOS's control.
RTOS Management: In Zephyr (and other RTOSes), the kernel tightly
manages the interrupt and exception state.
Direct manipulation of ICSR's volatile bits could conflict with the
RTOS's internal state machine, leading to crashes or unpredictable
behavior.
3. CFSR (Configurable Fault Status Register)
Motivation for Exclusion: This is a read-only status register that
reports the current state of Memory Management, Bus Fault, and Usage
Faults. It's used by fault handlers to determine the cause of a fault.
4. HFSR (HardFault Status Register)
Motivation for Exclusion: Similar to CFSR, this is a read-only status
register that reports the current state of HardFaults. It's for
reporting, not for configuration or restoration.
5. DFSR (Debug Fault Status Register)
Motivation for Exclusion: This is a read-only status register that
reports debug-related faults. It's primarily used by debuggers and
is not part of the application's runtime context to be saved/restored.
6. MMFAR (MemManage Fault Address Register)
Motivation for Exclusion: This is a read-only register that stores the
address that caused a Memory Management fault. It's a diagnostic
register, not a configurable parameter.
7. BFAR (BusFault Address Register)
Motivation for Exclusion: Similar to MMFAR, this is a read-only
register that stores the address that caused a BusFault. It's a
diagnostic register.
8. AFSR (Auxiliary Fault Status Register)
Motivation for Exclusion: This register is implementation-defined and
read-only.
Signed-off-by: Michele Sardo <msmttchr@gmail.com>
a90a47b1c9