1 | ////////////////////////////////////////////////////////////////////////////////// |
---|
2 | // File : barrier.c |
---|
3 | // Date : 01/04/2012 |
---|
4 | // Author : alain greiner |
---|
5 | // Copyright (c) UPMC-LIP6 |
---|
6 | /////////////////////////////////////////////////////////////////////////////////// |
---|
7 | // The barrier.c and barrier.h files are part of the GIET nano-kernel. |
---|
8 | // This user-level library provides a synchronisation service between several |
---|
9 | // tasks sharing the same address space in a parallel multi-tasks application. |
---|
10 | // |
---|
11 | // The barrier_init(), and the barrier_wait() functions do not require a syscall. |
---|
12 | // The barrier itself must have been allocated in a non cacheable segment, |
---|
13 | // if the platform does not provide hardwate cache coherence. |
---|
14 | // |
---|
15 | // ALL barriers must be defined in the mapping_info data structure, |
---|
16 | // to be initialised by the GIET in the boot phase. |
---|
17 | // The vobj_get_vbase() system call (defined in stdio.c and stdio.h files) |
---|
18 | // can be used to get the virtual base address of the barrier from it's name. |
---|
19 | /////////////////////////////////////////////////////////////////////////////////// |
---|
20 | |
---|
21 | #include <barrier.h> |
---|
22 | |
---|
23 | /////////////////////////////////////////////////////////////////////////////////// |
---|
24 | // barrier_init() |
---|
25 | // This function makes a cooperative initialisation of the barrier: |
---|
26 | // several tasks try to initialize the barrier, but the initialisation |
---|
27 | // is done by only one task, using LL/SC instructions. |
---|
28 | /////////////////////////////////////////////////////////////////////////////////// |
---|
29 | void barrier_init( giet_barrier_t* barrier, |
---|
30 | unsigned int value ) |
---|
31 | { |
---|
32 | unsigned int* pinit = (unsigned int*)&barrier->init; |
---|
33 | unsigned int* pcount = (unsigned int*)&barrier->count; |
---|
34 | |
---|
35 | // parallel initialisation using atomic instructions LL/SC |
---|
36 | // inputs : pinit, pcount, value |
---|
37 | // no output |
---|
38 | asm volatile ("_barrier_init_test: \n" |
---|
39 | "ll $2, 0(%0) \n" /* read initial value */ |
---|
40 | "bnez $2, _barrier_init_done \n" |
---|
41 | "move $3, %2 \n" |
---|
42 | "sc $3, 0(%0) \n" /* write initial value */ |
---|
43 | "beqz $3, _barrier_init_test \n" |
---|
44 | "move $3, %2 \n" |
---|
45 | "sw $3, 0(%1) \n" /* write count */ |
---|
46 | "_barrier_init_done: \n" |
---|
47 | :: "r"(pinit), "r"(pcount), "r"(value) |
---|
48 | : "$2", "$3"); |
---|
49 | } |
---|
50 | /////////////////////////////////////////////////////////////////////////////////// |
---|
51 | // barrier_wait() |
---|
52 | // This blocking function uses LL/SC to decrement the barrier's counter. |
---|
53 | // Then, it uses a busy_waiting mechanism if it is not the last. |
---|
54 | // (because the GIET does not support dynamic task scheduling/descheduling) |
---|
55 | /////////////////////////////////////////////////////////////////////////////////// |
---|
56 | void barrier_wait( giet_barrier_t* barrier ) |
---|
57 | { |
---|
58 | unsigned int* pcount = (unsigned int*)&barrier->count; |
---|
59 | unsigned int maxcount = barrier->init; |
---|
60 | unsigned int count; |
---|
61 | |
---|
62 | // parallel decrement barrier counter using atomic instructions LL/SC |
---|
63 | // - input : pointer on the barrier counter |
---|
64 | // - output : counter value |
---|
65 | asm volatile ("_barrier_decrement: \n" |
---|
66 | "ll %0, 0(%1) \n" |
---|
67 | "addi $3, %0, -1 \n" |
---|
68 | "sc $3, 0(%1) \n" |
---|
69 | "beqz $3, _barrier_decrement \n" |
---|
70 | : "=&r"(count) |
---|
71 | : "r"(pcount) |
---|
72 | : "$2", "$3"); |
---|
73 | |
---|
74 | // the last task re-initializes the barrier counter to the max value, |
---|
75 | // waking up all other waiting tasks |
---|
76 | |
---|
77 | if (count == 1) // last task |
---|
78 | { |
---|
79 | *pcount = maxcount; |
---|
80 | } |
---|
81 | else // other tasks busy-wait |
---|
82 | { |
---|
83 | while (*pcount != maxcount) asm volatile ("nop"); |
---|
84 | } |
---|
85 | } |
---|
86 | |
---|