這個問題應是斷網後，再次聯網前計算wpa_config_update_psk所致，這個會進行PBKDF2-SHA1 × 4096 次的運算，耗時約1.5s。如果想要省去這些時間，需要對這個函數做一下改進，當ssid的信息不變的時候，不重複計算此值。修改如下（計算前比較一下，如果信息不變不重新計算；但是第一次的計算是避免不了的，所以計算後要保存一下相關值）：

ifndef CONFIG_NO_PBKDF2

/*

• Cache last PBKDF2 result. wm_wifi_connect() does remove_network all +
• add_network on every retry, which allocates a new wpa_ssid and would
• otherwise re-run 4096-iteration PBKDF2 (~1s on MCU) for the same
• SSID/passphrase. Iteration count itself cannot be reduced (IEEE 802.11i).

*/
struct wpa_psk_cache {

int valid;
size_t ssid_len;
u8 ssid[SSID_MAX_LEN];
char passphrase[64];
u8 psk[PMK_LEN];

};

static struct wpa_psk_cache g_wpa_psk_cache;

endif / CONFIG_NO_PBKDF2 /

/**

• wpa_config_update_psk - Update WPA PSK based on passphrase and SSID
• @ssid: Pointer to network configuration data

*

• This function must be called to update WPA PSK when either SSID or the
• passphrase has changed for the network configuration.

*/
void wpa_config_update_psk(struct wpa_ssid *ssid)
{

ifndef CONFIG_NO_PBKDF2

if (ssid && ssid->passphrase && ssid->ssid_len <= SSID_MAX_LEN &&
    g_wpa_psk_cache.valid &&
    g_wpa_psk_cache.ssid_len == ssid->ssid_len &&
    os_memcmp(g_wpa_psk_cache.ssid, ssid->ssid, ssid->ssid_len) == 0 &&
    os_strcmp(g_wpa_psk_cache.passphrase, ssid->passphrase) == 0) {
    os_memcpy(ssid->psk, g_wpa_psk_cache.psk, PMK_LEN);
    ssid->psk_set = 1;
    wm_log_info("wpa_config_update_psk cache hit");
    wm_log_info("wpa_config_update_psk end");
    return;
}

pbkdf2_sha1(ssid->passphrase, ssid->ssid, ssid->ssid_len, 4096,
        ssid->psk, PMK_LEN);
wpa_hexdump_key(MSG_MSGDUMP, "PSK (from passphrase)",
        ssid->psk, PMK_LEN);
ssid->psk_set = 1;

if (ssid->passphrase && ssid->ssid_len <= SSID_MAX_LEN &&
    os_strlen(ssid->passphrase) < sizeof(g_wpa_psk_cache.passphrase)) {
    g_wpa_psk_cache.ssid_len = ssid->ssid_len;
    os_memcpy(g_wpa_psk_cache.ssid, ssid->ssid, ssid->ssid_len);
    os_strlcpy(g_wpa_psk_cache.passphrase, ssid->passphrase,
           sizeof(g_wpa_psk_cache.passphrase));
    os_memcpy(g_wpa_psk_cache.psk, ssid->psk, PMK_LEN);
    g_wpa_psk_cache.valid = 1;
}

endif / CONFIG_NO_PBKDF2 /

}