audk/ArmPlatformPkg/Sec/Helper.S

#========================================================================================
#  Copyright (c) 2011-2012, ARM Limited. All rights reserved.
#  
#  This program and the accompanying materials                          
#  are licensed and made available under the terms and conditions of the BSD License         
#  which accompanies this distribution.  The full text of the license may be found at        
#  http:#opensource.org/licenses/bsd-license.php                                            
#
#  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,                     
#  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.             
#
#=======================================================================================

#start of the code section
.text 
.align 3

GCC_ASM_EXPORT(return_from_exception)
GCC_ASM_EXPORT(enter_monitor_mode)
GCC_ASM_EXPORT(copy_cpsr_into_spsr)
GCC_ASM_EXPORT(set_non_secure_mode)

# arg0: Secure Monitor mode stack
ASM_PFX(enter_monitor_mode):
    mov     r2, lr                      @ Save current lr

    mrs     r1, cpsr                    @ Save current mode (SVC) in r1
    bic     r3, r1, #0x1f               @ Clear all mode bits
    orr     r3, r3, #0x16               @ Set bits for Monitor mode
    msr     cpsr_cxsf, r3               @ We are now in Monitor Mode

    mov     sp, r0                      @ Use the passed sp
    mov     lr, r2                      @ Use the same lr as before
    
    msr     spsr_cxsf, r1               @ Use saved mode for the MOVS jump to the kernel
    bx      lr

# We cannot use the instruction 'movs pc, lr' because the caller can be written either in ARM or Thumb2 assembler.
# When we will jump into this function, we will set the CPSR flag to ARM assembler. By copying directly 'lr' into
# 'pc'; we will not change the CPSR flag and it will crash.
# The way to fix this limitation is to do the movs into the ARM assmbler code and then do a 'bx'.
ASM_PFX(return_from_exception):
    ldr     lr, returned_exception

    #The following instruction breaks the code.
    #movs    pc, lr
    mrs     r2, cpsr
    bic     r2, r2, #0x1f
    orr     r2, r2, #0x13
    msr     cpsr_c, r2

returned_exception:                           @ We are now in non-secure state
    bx      r0

# Save the current Program Status Register (PSR) into the Saved PSR
ASM_PFX(copy_cpsr_into_spsr):
    mrs     r0, cpsr
    msr     spsr_cxsf, r0
    bx      lr

# Set the Non Secure Mode
ASM_PFX(set_non_secure_mode):
    push    { r1 }
    and	    r0, r0, #0x1f     @ Keep only the mode bits
    mrs     r1, spsr          @ Read the spsr
    bic     r1, r1, #0x1f     @ Clear all mode bits
    orr	    r1, r1, r0
    msr     spsr_cxsf, r1     @ write back spsr (may have caused a mode switch)
    isb
    pop     { r1 }
    bx      lr                @ return (hopefully thumb-safe!)

dead:
    b       dead
    
ASM_FUNCTION_REMOVE_IF_UNREFERENCED