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authorWojtek Kosior <kwojtus@protonmail.com>2019-12-30 23:23:30 +0100
committerWojtek Kosior <kwojtus@protonmail.com>2019-12-30 23:23:30 +0100
commit6421fd0e09448ed9134dab3338cfdda848a8eeaa (patch)
tree4ba1268ea047c99f9cfa9d789964b36ab05f725c
parent7efc53fe9a593eab08400c18e6b0def30c17e684 (diff)
downloadrpi-MMU-example-6421fd0e09448ed9134dab3338cfdda848a8eeaa.tar.gz
rpi-MMU-example-6421fd0e09448ed9134dab3338cfdda848a8eeaa.zip
preserve atags for stage2
-rw-r--r--kernel_stage1.S105
1 files changed, 103 insertions, 2 deletions
diff --git a/kernel_stage1.S b/kernel_stage1.S
index 1e0f614..dd3e6fd 100644
--- a/kernel_stage1.S
+++ b/kernel_stage1.S
@@ -20,8 +20,105 @@ _boot:
// occur when the main core overwrites this part of memory
wfe
+ // we'll use the size of stage1 to determine where we have free
+ // space after it. We'll then copy our atags/fdt there, so
+ // it doesn't get overwritten by stage2 we deploy at 0x0
+atags_magic:
+ .word 0x54410001
+
proceed:
- // copy stage2 of the kernel to address 0x0
+ // load the second word of structure passed to us through r2;
+ // if it's atags, it's second word should be the magic number
+ // Btw, location of ATAGS is always 0x100.
+ ldr r3, [r2, #4]
+ adr r4, atags_magic
+ ldr r4, [r4]
+
+ // compare second word of assumed atags with magic number
+ // to see, if it's really atags and not sth else (i.e. fdt)
+ cmp r3, r4
+
+ // normally at start r0 contains value 0;
+ // value 3 in r0 would tell stage2 code, we found no atags :(
+ movne r0, #3
+ bne stage2_blob_copying
+
+ // if atags was found, copying of it takes place here
+
+ // the following loop finds, where atags ends
+ // r3 shall point to currently looked-at tag
+ mov r3, r2
+
+find_end_of_atags_loop:
+ // load first word of tag header to r4 (it contains tag size)
+ ldr r4, [r3]
+ // make r3 point at the next tag (by adding 4*tag_size to it)
+ add r3, r4, lsl #2
+
+ // load second word of tag header to r5 (it contains tag type)
+ ldr r5, [r3, #4]
+
+ // if tag value is 0, it is the last tag
+ cmp r5, #0
+ bne find_end_of_atags_loop
+
+ add r3, #8 // make r3 point at the end of last tag
+ sub r3, r2 // get atags size in r3
+
+ // at this pont r2 and r3 point at start and size of atags,
+ // respectively; now we'll compute, where we're going to have
+ // free space to put atags in; we want to put atags either
+ // right after our blob or, if if it doesn't fit between
+ // blob end and the address stage1 is loaded at, after stage1
+
+ // get blob size to r5
+ adr r5, blob_size
+ ldr r5, [r5]
+
+ // we could only copy atags to a 4-aligned address
+ mov r6, #4
+ bl aling_r5_to_r6
+
+ // compute where atags copied right after blob would end
+ add r6, r5, r3
+ // we can only overwrite stuff before the copying loop
+ adr r7, copy_atags_loop
+ cmp r6, r7
+ ble copy_atags
+
+ // atags wouldn't fit - use memory after stage1 as destination
+ adr r5, _boot
+ adr r6, stage1_size
+ ldr r6, [r6]
+ add r5, r6
+ mov r6, #4
+ bl aling_r5_to_r6
+
+copy_atags:
+ // now copy atags (r2 - atags start; r3 - atags size;
+ // r5 - destination; r4 - iterator; r6 - buffor)
+ mov r4, #0
+
+copy_atags_loop:
+ ldr r6, [r2, r4]
+ str r6, [r5, r4]
+ add r4, #4
+ cmp r4, r3
+ blo copy_atags_loop
+
+ mov r2, r5 // place the new atags address in r2
+ b stage2_blob_copying // atags stuff done; proceed
+
+// mini-function, that does what the label says; clobbers r7
+aling_r5_to_r6:
+ sub r5, #1
+ sub r7, r6, #1
+ bic r5, r7
+ add r5, r6
+ mov pc, lr
+
+
+stage2_blob_copying: // copy stage2 of the kernel to address 0x0
// first, load address of stage2_start to r3 (a PIC way)
adr r3, stage2_start
@@ -47,6 +144,8 @@ proceed:
blob_size:
.word stage2_end - stage2_start
+stage1_size:
+ .word stage1_end - _boot
.align 4
stage2_start:
@@ -61,7 +160,9 @@ loop:
add r6, r6, #4
cmp r6, r5
blo loop
-
+
// Call stage2 of the kernel (branch to 0x0,
// which is the reset handler).
bx r4
+
+stage1_end: