diff options
| -rw-r--r-- | openpicc/os/boot/boot.s | 101 |
1 files changed, 42 insertions, 59 deletions
diff --git a/openpicc/os/boot/boot.s b/openpicc/os/boot/boot.s index 0371b6e..2063a48 100644 --- a/openpicc/os/boot/boot.s +++ b/openpicc/os/boot/boot.s @@ -250,74 +250,57 @@ my_fiq_handler: ldr r11, =ssc_tx_fiq_fdt_cdiv ldr r11, [r11] /* r11 == ssc_tx_fiq_fdt_cdiv */ -/* Problem: LDR from the timer still takes too long and causes us to miss the exact time. - * Strategy: Spin on TC2 till we are very near the actual time. Then load the timer value, calculate - * the difference to the target, then do a countdown spin without reading the timer. +/* Problem: LDR from the timer and loop still take too long and cause us to miss the exact time. + * (One load-from-timer,compare,jump-back-if-less cycle are 7 CPU cycles, which are 2 carrier cycles.) + * Strategy: Spin on TC2 till 3 or 4 carrier cycles before the actual time. Then go into an unrolled + * 'loop' for the remaining time. (load-from-timer,compare,jump-forward-if-greater-or-equal are 5 CPU + * cycles if the condition is not reached.) * * At 47.923200 MHz 7 processor cycles are 2 carrier cycles of the 13.56MHz carrier */ - .equ SUB_TIME, 20 /* subtract 20 carrier cycles == 70 processor cycles */ - .equ ADD_TIME, (70-30) /* Add x processor cycles */ + .equ SUB_TIME, 4 /* subtract 4 carrier cycles == 14 processor cycles */ mov r8, #SUB_TIME - sub r11, r11, r8 + sub r11, r11, r8 /* r11 == fdt_cdiv-SUB_TIME */ -.wait_for_fdt_cdiv: +.wait_for_fdt_cdiv: ldr r8, [r12, #TC2_CV] cmp r8, r11 - bmi .wait_for_fdt_cdiv /* spin while TC2.CV is less fdt_cdiv-SUB_TIM */ - - ldr r8, [r12, #TC2_CV] - sub r11, r11, r8 /* r11 == fdt_cdiv-SUB_TIME - TC2.CV */ - - mov r8, #0x07 - mul r11, r8, r11 /* r11 = r11 * 7 */ - mov r11, r11, ASR #1 /* r11 = r11 / 2 */ - - mov r8, #ADD_TIME - adds r11, r11, r8 /* r11 = r11 + ADD_TIME */ - - bmi .wait_zero - -/* The following contraption is designed to compensate for the fact - that the loop below will only be able to count CPU cycles with a - precision of 4 cycles. Constraint: Since we want to keep the value - of r11 there's only one free register: r8.*/ - mov r8, #0x1 - tst r11, r8 - beq .wait_zero_or_two - bne .wait_one_or_three - -/* relative timing ------+ All code paths have the same number of CPU cycles, except - V for the relative timing given in these comments */ -.wait_zero_or_two: /* +0 */ - mov r8, #0x2 - tst r11, r8 - beq .wait_zero /* +0 */ - nop - bne .wait_two /* +2 */ - -.wait_one_or_three: /* +1 */ - mov r8, #0x2 - tst r11, r8 - beq .wait_one /* +0 */ - nop - bne .wait_three /* +2 */ - - nop /* Don't know if this nop is needed. Just add to make sure that nobody can assume that the "bne .wait_three" is no branch at all. */ - -.wait_three: -.wait_two: -.wait_one: -.wait_zero: - - mov r11, r11, ASR #2 /* r11 = r11 / 4 (4 is the number of cycles per loop run below) */ - - mov r8, #1 -.wait_for_my_time: - subs r11, r11, r8 - bge .wait_for_my_time + blt .wait_for_fdt_cdiv /* spin while TC2.CV is less fdt_cdiv-SUB_TIME */ + mov r8, #SUB_TIME + add r11, r11, r8 /* r11 == fdt_cdiv */ + +/* Seven copies of the loop contents, covering for 35 CPU cycles, or 10 carrier cycles */ + ldr r8, [r12, #TC2_CV] + cmp r8, r11 + bge .fdt_expired /* jump forward if TC2.CV is greater than or equal fdt_cdiv */ + + ldr r8, [r12, #TC2_CV] + cmp r8, r11 + bge .fdt_expired + + ldr r8, [r12, #TC2_CV] + cmp r8, r11 + bge .fdt_expired + + ldr r8, [r12, #TC2_CV] + cmp r8, r11 + bge .fdt_expired + + ldr r8, [r12, #TC2_CV] + cmp r8, r11 + bge .fdt_expired + + ldr r8, [r12, #TC2_CV] + cmp r8, r11 + bge .fdt_expired + + ldr r8, [r12, #TC2_CV] + cmp r8, r11 + bge .fdt_expired + +.fdt_expired: str r9, [r12, #TC_CCR] /* SWTRG on TC0 */ ldr r11, =ssc_tx_fiq_fdt_ssc |