ath9k: Check for errors when reading SREV register

Right now, if an error is encountered during the SREV register
read (i.e. an EIO in ath9k_regread()), that error code gets
passed all the way to __ath9k_hw_init(), where it is visible
during the "Chip rev not supported" message.

    ath9k_htc 1-1.4:1.0: ath9k_htc: HTC initialized with 33 credits
    ath: phy2: Mac Chip Rev 0x0f.3 is not supported by this driver
    ath: phy2: Unable to initialize hardware; initialization status: -95
    ath: phy2: Unable to initialize hardware; initialization status: -95
    ath9k_htc: Failed to initialize the device

Check for -EIO explicitly in ath9k_hw_read_revisions() and return
a boolean based on the success of the operation. Check for that in
__ath9k_hw_init() and abort with a more debugging-friendly message
if reading the revisions wasn't successful.

    ath9k_htc 1-1.4:1.0: ath9k_htc: HTC initialized with 33 credits
    ath: phy2: Failed to read SREV register
    ath: phy2: Could not read hardware revision
    ath: phy2: Unable to initialize hardware; initialization status: -95
    ath: phy2: Unable to initialize hardware; initialization status: -95
    ath9k_htc: Failed to initialize the device

This helps when debugging by directly showing the first point of
failure and it could prevent possible errors if a 0x0f.3 revision
is ever supported.
Signed-off-by: Tim Schumacher <timschumi@gmx.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ath/ath9k/hw.c

@@ -252,8 +252,9 @@ void ath9k_hw_get_channel_centers(struct ath_hw *ah,
 /* Chip Revisions */
 /******************/
 
-static void ath9k_hw_read_revisions(struct ath_hw *ah)
+static bool ath9k_hw_read_revisions(struct ath_hw *ah)
 {
+	u32 srev;
 	u32 val;
 
 	if (ah->get_mac_revision)
@@ -269,25 +270,33 @@ static void ath9k_hw_read_revisions(struct ath_hw *ah)
 			val = REG_READ(ah, AR_SREV);
 			ah->hw_version.macRev = MS(val, AR_SREV_REVISION2);
 		}
-		return;
+		return true;
 	case AR9300_DEVID_AR9340:
 		ah->hw_version.macVersion = AR_SREV_VERSION_9340;
-		return;
+		return true;
 	case AR9300_DEVID_QCA955X:
 		ah->hw_version.macVersion = AR_SREV_VERSION_9550;
-		return;
+		return true;
 	case AR9300_DEVID_AR953X:
 		ah->hw_version.macVersion = AR_SREV_VERSION_9531;
-		return;
+		return true;
 	case AR9300_DEVID_QCA956X:
 		ah->hw_version.macVersion = AR_SREV_VERSION_9561;
-		return;
+		return true;
 	}
 
-	val = REG_READ(ah, AR_SREV) & AR_SREV_ID;
+	srev = REG_READ(ah, AR_SREV);
+
+	if (srev == -EIO) {
+		ath_err(ath9k_hw_common(ah),
+			"Failed to read SREV register");
+		return false;
+	}
+
+	val = srev & AR_SREV_ID;
 
 	if (val == 0xFF) {
-		val = REG_READ(ah, AR_SREV);
+		val = srev;
 		ah->hw_version.macVersion =
 			(val & AR_SREV_VERSION2) >> AR_SREV_TYPE2_S;
 		ah->hw_version.macRev = MS(val, AR_SREV_REVISION2);
@@ -306,6 +315,8 @@ static void ath9k_hw_read_revisions(struct ath_hw *ah)
 		if (ah->hw_version.macVersion == AR_SREV_VERSION_5416_PCIE)
 			ah->is_pciexpress = true;
 	}
+
+	return true;
 }
 
 /************************************/
@@ -559,7 +570,10 @@ static int __ath9k_hw_init(struct ath_hw *ah)
 	struct ath_common *common = ath9k_hw_common(ah);
 	int r = 0;
 
-	ath9k_hw_read_revisions(ah);
+	if (!ath9k_hw_read_revisions(ah)) {
+		ath_err(common, "Could not read hardware revisions");
+		return -EOPNOTSUPP;
+	}
 
 	switch (ah->hw_version.macVersion) {
 	case AR_SREV_VERSION_5416_PCI: