#littlefs原理分析#[六]磨损均衡 精华

// 当将缓存中的数据回写到磁盘时，检测坏块
lfs_bd_flush(lfs_t *lfs,
|       lfs_cache_t *pcache, lfs_cache_t *rcache, bool validate) 
|   ...
|
|   // 调用lfs_bd_cmp比较磁盘上的数据是否与写入的数据相同
|   // 如果不同则可能遇到了坏块
|-> if (validate) {
|       lfs_bd_cmp(lfs,
|               NULL, rcache, diff,
|               pcache->block, pcache->off, pcache->buffer, diff); 
|   }
|
|-> ...
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lfs_file_flush(lfs_t *lfs, lfs_file_t *file)
|-> ...
|
|-> while (true) {
|       // 调用lfs_bd_flush写入数据，并比较数据是否写入正确
|       int err = lfs_bd_flush(lfs, &file->cache, &lfs->rcache, true);
|       if (err) {
|           // 检测到坏块则跳转到relocate
|           if (err == LFS_ERR_CORRUPT) {
|               goto relocate;
|           }
|           return err;
|       }
|       break;
|
|   relocate:
|       // 重新分配块并再次进行写入操作
|       LFS_DEBUG("Bad block at 0x%"PRIx32, file->block);
|       err = lfs_file_relocate(lfs, file);
|       if (err) {
|           return err;
|       }
|   }
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struct lfs_free {
    lfs_block_t off;     // lookahead所有block整体的偏移
    lfs_block_t size;    // lookahead中块的总数
    lfs_block_t i;       // 在lookahead_size中的索引，表示当前位于第几个block
    lfs_block_t ack;     // 所有剩余空闲block个数
    uint32_t *buffer;    // lookahead的bitmap块管理缓存区
} free;
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lfs_alloc(lfs_t *lfs, lfs_block_t *block)
|   ...
|
|   // 当lookahead buffer中没有空闲块时，需进行扫描
|
|   // 1. 推移lookahead buffer
|-> lfs->free.off = (lfs->free.off + lfs->free.size)
|            % lfs->cfg->block_count;
|   lfs->free.size = lfs_min(8*lfs->cfg->lookahead_size, lfs->free.ack);
|   lfs->free.i = 0;
|
|   // 2. 将lookahead buffer清0
|-> memset(lfs->free.buffer, 0, lfs->cfg->lookahead_size);
|
|   // 3. 遍历文件系统进行扫描和查找
|-> lfs_fs_rawtraverse(lfs, lfs_alloc_lookahead, lfs, true);
|
|-> ...
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// lfs_fs_rawtraverse函数传入到lfs_alloc_lookahead函数的参数
// 分别为lfs结构体指针p，和块号block
lfs_alloc_lookahead(void *p, lfs_block_t block)
|-> lfs_t *lfs = (lfs_t*)p;
|
|   // 获取块号相对lookahead buffer的偏移
|-> lfs_block_t off = ((block - lfs->free.off)
|           + lfs->cfg->block_count) % lfs->cfg->block_count;
|
|   // 若该块处于lookahead buffer所管理的范围内，
|   // 则设置bitmap对应位，表示该块已分配
|-> if (off < lfs->free.size) {
|       lfs->free.buffer[off / 32] |= 1U << (off % 32);
|   }
|   return 0;
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boot...         lookahead:
                fs blocks: fffff9fffffffffeffffffffffff0000
scanning...     lookahead: fffff9ff
                fs blocks: fffff9fffffffffeffffffffffff0000
alloc = 21      lookahead: fffffdff
                fs blocks: fffffdfffffffffeffffffffffff0000
alloc = 22      lookahead: ffffffff
                fs blocks: fffffffffffffffeffffffffffff0000
scanning...     lookahead:         fffffffe
                fs blocks: fffffffffffffffeffffffffffff0000
alloc = 63      lookahead:         ffffffff
                fs blocks: ffffffffffffffffffffffffffff0000
scanning...     lookahead:         ffffffff
                fs blocks: ffffffffffffffffffffffffffff0000
scanning...     lookahead:                 ffffffff
                fs blocks: ffffffffffffffffffffffffffff0000
scanning...     lookahead:                         ffff0000
                fs blocks: ffffffffffffffffffffffffffff0000
alloc = 112     lookahead:                         ffff8000
                fs blocks: ffffffffffffffffffffffffffff8000
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lfs_mount(lfs_t *lfs, const struct lfs_config *cfg)
|-> lfs_rawmount(lfs_t *lfs, const struct lfs_config *cfg)
    |-> ...
    |
    |   // 1. 计算随机数
    |-> lfs_dir_fetchmatch(...)
        |-> ...
        |
        |   // 使用crc计算随机数
        |-> lfs->seed = lfs_crc(lfs->seed, &crc, sizeof(crc));
        |
        |-> ...
    |
    |   // 2. 随机对lookahead buffer进行偏移
    |-> lfs->free.off = lfs->seed % lfs->cfg->block_count;
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lfs_dir_compact(lfs_t *lfs,
|       lfs_mdir_t *dir, const struct lfs_mattr *attrs, int attrcount,
|       lfs_mdir_t *source, uint16_t begin, uint16_t end) 
|-> ...
|
|   // revision count为block_cycles的整数倍时，进行元数据对应块的重分配
|-> if (lfs->cfg->block_cycles > 0 &&
|       (dir->rev % ((lfs->cfg->block_cycles+1)|1) == 0)) {
|       ...
|
|       // we're writing too much, time to relocate
|       tired = true;
|       goto relocate;
|   }
|
|-> ...
|
|   relocate:
|-> ...
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