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https://github.com/Atmosphere-NX/Atmosphere
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123 lines
3.4 KiB
C
123 lines
3.4 KiB
C
#ifndef EXOSPHERE_SYNCHRONIZATION_H
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#define EXOSPHERE_SYNCHRONIZATION_H
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#include <stdatomic.h>
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#include "utils.h"
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/* Simple atomics driver for Exosphere. */
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typedef struct {
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struct {
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uint8_t ticket_number : 7;
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uint8_t is_entering : 1;
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} customers[4];
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} critical_section_t;
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static inline void __dsb_sy(void) {
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__asm__ __volatile__ ("dsb sy" ::: "memory");
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}
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static inline void __dsb_ish(void) {
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__asm__ __volatile__ ("dsb ish" ::: "memory");
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}
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static inline void __dmb_sy(void) {
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__asm__ __volatile__ ("dmb sy" ::: "memory");
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}
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static inline void __isb(void) {
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__asm__ __volatile__ ("isb" ::: "memory");
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}
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static inline void __sev(void) {
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__asm__ __volatile__ ("sev");
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}
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static inline void __sevl(void) {
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__asm__ __volatile__ ("sevl");
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}
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static inline void __wfe(void) {
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__asm__ __volatile__ ("wfe");
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}
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/* Acquire a lock. */
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static inline void lock_acquire(atomic_flag *flag) {
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while (atomic_flag_test_and_set_explicit(flag, memory_order_acquire)) {
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/* Wait to acquire lock. */
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}
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}
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/* Release a lock. */
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static inline void lock_release(atomic_flag *flag) {
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atomic_flag_clear_explicit(flag, memory_order_release);
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}
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/* Try to acquire a lock. */
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static inline bool lock_try_acquire(atomic_flag *flag) {
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return !atomic_flag_test_and_set_explicit(flag, memory_order_acquire);
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}
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/*
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Enter a critical section, using the Lamport's bakery algorithm.
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https://en.wikipedia.org/wiki/Lamport%27s_bakery_algorithm
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This is invoked on warmboot before the MMU is turned on, therefore
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exclusive load/store instructions can't be used.
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Note: Nintendo has tried to implement that algorithm, but it seems that
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they didn't understand how it works, and their implementation
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is therefore a complete failure: in particular, the "ticket number" is
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always the same (core0 will always enter the critical section before all the cores,
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and so on...), and thus there can be starvation, etc.
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Nintendo, wtf.
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*/
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ALINLINE static inline unsigned int critical_section_enter(volatile critical_section_t *critical_section) {
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unsigned int id = get_core_id();
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uint8_t my_ticket_number = 0;
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uint8_t tmp;
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critical_section->customers[id].is_entering = 1;
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for (unsigned int i = 0; i < 4; i++) {
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tmp = critical_section->customers[id].ticket_number;
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my_ticket_number = tmp > my_ticket_number ? tmp : my_ticket_number;
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}
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critical_section->customers[id].ticket_number = ++my_ticket_number;
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critical_section->customers[id].is_entering = 0;
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__dsb_sy();
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__sev();
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for (unsigned int i = 0; i < 4; i++) {
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__sevl();
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do {
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__wfe();
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} while (critical_section->customers[i].is_entering);
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__sevl();
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do {
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__wfe();
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tmp = critical_section->customers[i].ticket_number;
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} while (tmp && (tmp < my_ticket_number || (tmp == my_ticket_number && i < id)));
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}
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__dmb_sy();
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return id;
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}
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/*
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Leaves a critical section, using the Lamport's bakery algorithm (see above).
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Note: Nintendo failed even that: they're clearing the entire critical section state
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instead of just the counter associated to the current core.
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Nintendo, wtf.
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*/
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ALINLINE static inline void critical_section_leave(volatile critical_section_t *critical_section) {
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critical_section->customers[get_core_id()].ticket_number = 0;
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__dsb_sy();
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__sev();
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}
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#endif
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