ARM: PSCI: Allocate PSCI stack in secure stack section

Now that we have a secure stack section that guarantees usable memory,
allocate the PSCI stacks in that section.

Also add a diagram detailing how the stacks are placed in memory.

Reserved space for the target PC remains unchanged. This should be
moved to global variables within a secure data section in the future.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

arch/arm/cpu/armv7/psci.S

@@ -196,18 +196,28 @@ ENTRY(psci_cpu_off_common)
 	bx	lr
 ENDPROC(psci_cpu_off_common)
 
-@ expects CPU ID in r0 and returns stack top in r0
+@ The stacks are allocated in reverse order, i.e.
+@ the stack for CPU0 has the highest memory address.
+@
+@ --------------------  __secure_stack_end
+@ |  CPU0 target PC  |
+@ |------------------|
+@ |                  |
+@ |    CPU0 stack    |
+@ |                  |
+@ |------------------|  __secure_stack_end - 1KB
+@ |        .         |
+@ |        .         |
+@ |        .         |
+@ |        .         |
+@ --------------------  __secure_stack_start
+@
+@ This expects CPU ID in r0 and returns stack top in r0
 ENTRY(psci_get_cpu_stack_top)
-	mov	r3, #0x400			@ 1kB of stack per CPU
-	mul	r0, r0, r3
-
-	ldr	r3, =psci_text_end		@ end of monitor text
-	add	r3, r3, #0x2000			@ Skip two pages
-	lsr	r3, r3, #12			@ Align to start of page
-	lsl	r3, r3, #12
-	sub	r3, r3, #4			@ reserve 1 word for target PC
-	sub	r0, r3, r0			@ here's our stack!
-
+	@ stack top = __secure_stack_end - (cpuid << ARM_PSCI_STACK_SHIFT)
+	ldr	r3, =__secure_stack_end
+	sub	r0, r3, r0, LSL #ARM_PSCI_STACK_SHIFT
+	sub	r0, r0, #4		@ Save space for target PC
 	bx	lr
 ENDPROC(psci_get_cpu_stack_top)