ath9k: Document keycache operations

There are number of small details about the keycache operations that
are very easy to miss (and forget), so better include detailed
comments in ath9k_hw_set_keycache_entry() to avoid having to figure
out this every time when having to touch this area.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 73 insertions, 3 deletions

diff --git a/drivers/net/wireless/ath9k/hw.c b/drivers/net/wireless/ath9k/hw.c
index 2acbb84dc2ba..5672f43e58e2 100644
--- a/drivers/net/wireless/ath9k/hw.c
+++ b/drivers/net/wireless/ath9k/hw.c
@@ -2482,18 +2482,49 @@ bool ath9k_hw_set_keycache_entry(struct ath_hw *ah, u16 entry,
         if (k->kv_len <= LEN_WEP104)
                 key4 &= 0xff;
 
+        /*
+         * Note: Key cache registers access special memory area that requires
+         * two 32-bit writes to actually update the values in the internal
+         * memory. Consequently, the exact order and pairs used here must be
+         * maintained.
+         */
+
         if (keyType == AR_KEYTABLE_TYPE_TKIP && ATH9K_IS_MIC_ENABLED(ah)) {
                 u16 micentry = entry + 64;
 
+                /*
+                 * Write inverted key[47:0] first to avoid Michael MIC errors
+                 * on frames that could be sent or received at the same time.
+                 * The correct key will be written in the end once everything
+                 * else is ready.
+                 */
                 REG_WRITE(ah, AR_KEYTABLE_KEY0(entry), ~key0);
                 REG_WRITE(ah, AR_KEYTABLE_KEY1(entry), ~key1);
+
+                /* Write key[95:48] */
                 REG_WRITE(ah, AR_KEYTABLE_KEY2(entry), key2);
                 REG_WRITE(ah, AR_KEYTABLE_KEY3(entry), key3);
+
+                /* Write key[127:96] and key type */
                 REG_WRITE(ah, AR_KEYTABLE_KEY4(entry), key4);
                 REG_WRITE(ah, AR_KEYTABLE_TYPE(entry), keyType);
+
+                /* Write MAC address for the entry */
                 (void) ath9k_hw_keysetmac(ah, entry, mac);
 
                 if (ah->misc_mode & AR_PCU_MIC_NEW_LOC_ENA) {
+                        /*
+                         * TKIP uses two key cache entries:
+                         * Michael MIC TX/RX keys in the same key cache entry
+                         * (idx = main index + 64):
+                         * key0 [31:0] = RX key [31:0]
+                         * key1 [15:0] = TX key [31:16]
+                         * key1 [31:16] = reserved
+                         * key2 [31:0] = RX key [63:32]
+                         * key3 [15:0] = TX key [15:0]
+                         * key3 [31:16] = reserved
+                         * key4 [31:0] = TX key [63:32]
+                         */
                         u32 mic0, mic1, mic2, mic3, mic4;
 
                         mic0 = get_unaligned_le32(k->kv_mic + 0);
@@ -2501,45 +2532,84 @@ bool ath9k_hw_set_keycache_entry(struct ath_hw *ah, u16 entry,
                         mic1 = get_unaligned_le16(k->kv_txmic + 2) & 0xffff;
                         mic3 = get_unaligned_le16(k->kv_txmic + 0) & 0xffff;
                         mic4 = get_unaligned_le32(k->kv_txmic + 4);
+
+                        /* Write RX[31:0] and TX[31:16] */
                         REG_WRITE(ah, AR_KEYTABLE_KEY0(micentry), mic0);
                         REG_WRITE(ah, AR_KEYTABLE_KEY1(micentry), mic1);
+
+                        /* Write RX[63:32] and TX[15:0] */
                         REG_WRITE(ah, AR_KEYTABLE_KEY2(micentry), mic2);
                         REG_WRITE(ah, AR_KEYTABLE_KEY3(micentry), mic3);
+
+                        /* Write TX[63:32] and keyType(reserved) */
                         REG_WRITE(ah, AR_KEYTABLE_KEY4(micentry), mic4);
                         REG_WRITE(ah, AR_KEYTABLE_TYPE(micentry),
                                   AR_KEYTABLE_TYPE_CLR);
 
                 } else {
+                        /*
+                         * TKIP uses four key cache entries (two for group
+                         * keys):
+                         * Michael MIC TX/RX keys are in different key cache
+                         * entries (idx = main index + 64 for TX and
+                         * main index + 32 + 96 for RX):
+                         * key0 [31:0] = TX/RX MIC key [31:0]
+                         * key1 [31:0] = reserved
+                         * key2 [31:0] = TX/RX MIC key [63:32]
+                         * key3 [31:0] = reserved
+                         * key4 [31:0] = reserved
+                         *
+                         * Upper layer code will call this function separately
+                         * for TX and RX keys when these registers offsets are
+                         * used.
+                         */
                         u32 mic0, mic2;
 
                         mic0 = get_unaligned_le32(k->kv_mic + 0);
                         mic2 = get_unaligned_le32(k->kv_mic + 4);
+
+                        /* Write MIC key[31:0] */
                         REG_WRITE(ah, AR_KEYTABLE_KEY0(micentry), mic0);
                         REG_WRITE(ah, AR_KEYTABLE_KEY1(micentry), 0);
+
+                        /* Write MIC key[63:32] */
                         REG_WRITE(ah, AR_KEYTABLE_KEY2(micentry), mic2);
                         REG_WRITE(ah, AR_KEYTABLE_KEY3(micentry), 0);
+
+                        /* Write TX[63:32] and keyType(reserved) */
                         REG_WRITE(ah, AR_KEYTABLE_KEY4(micentry), 0);
                         REG_WRITE(ah, AR_KEYTABLE_TYPE(micentry),
                                   AR_KEYTABLE_TYPE_CLR);
                 }
+
+                /* MAC address registers are reserved for the MIC entry */
                 REG_WRITE(ah, AR_KEYTABLE_MAC0(micentry), 0);
                 REG_WRITE(ah, AR_KEYTABLE_MAC1(micentry), 0);
+
+                /*
+                 * Write the correct (un-inverted) key[47:0] last to enable
+                 * TKIP now that all other registers are set with correct
+                 * values.
+                 */
                 REG_WRITE(ah, AR_KEYTABLE_KEY0(entry), key0);
                 REG_WRITE(ah, AR_KEYTABLE_KEY1(entry), key1);
         } else {
+                /* Write key[47:0] */
                 REG_WRITE(ah, AR_KEYTABLE_KEY0(entry), key0);
                 REG_WRITE(ah, AR_KEYTABLE_KEY1(entry), key1);
+
+                /* Write key[95:48] */
                 REG_WRITE(ah, AR_KEYTABLE_KEY2(entry), key2);
                 REG_WRITE(ah, AR_KEYTABLE_KEY3(entry), key3);
+
+                /* Write key[127:96] and key type */
                 REG_WRITE(ah, AR_KEYTABLE_KEY4(entry), key4);
                 REG_WRITE(ah, AR_KEYTABLE_TYPE(entry), keyType);
 
+                /* Write MAC address for the entry */
                 (void) ath9k_hw_keysetmac(ah, entry, mac);
         }
 
-        if (ah->curchan == NULL)
-                return true;
-
         return true;
 }