net: sfp: fix high power modules without diagnostic monitoring

Commit 7cfa9c92d0 ("net: sfp: avoid power switch on address-change
modules") unintetionally changed the semantics for high power modules
without the digital diagnostics monitoring. We repeatedly attempt to
read the power status from the non-existing 0xa2 address in a futile
hope this failure is temporary:

[    8.856051] sfp sfp-eth3: module NTT              0000000000000000 rev 0000  sn 0000000000000000 dc 160408
[    8.865843] mvpp2 f4000000.ethernet eth3: switched to inband/1000base-x link mode
[    8.873469] sfp sfp-eth3: Failed to read EEPROM: -5
[    8.983251] sfp sfp-eth3: Failed to read EEPROM: -5
[    9.103250] sfp sfp-eth3: Failed to read EEPROM: -5

We previosuly assumed such modules were powered up in the correct mode,
continuing without further configuration as long as the required power
class was supported by the host.

Restore this behaviour, while preserving the intent of subsequent
patches to avoid the "Address Change Sequence not supported" warning
if we are not going to be accessing the DDM address.

Fixes: 7cfa9c92d0 ("net: sfp: avoid power switch on address-change modules")
Reported-by: 照山周一郎 <teruyama@springboard-inc.jp>
Tested-by: 照山周一郎 <teruyama@springboard-inc.jp>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/sfp.c

@@ -1641,17 +1641,20 @@ static int sfp_sm_probe_for_phy(struct sfp *sfp)
 static int sfp_module_parse_power(struct sfp *sfp)
 {
 	u32 power_mW = 1000;
+	bool supports_a2;
 
 	if (sfp->id.ext.options & cpu_to_be16(SFP_OPTIONS_POWER_DECL))
 		power_mW = 1500;
 	if (sfp->id.ext.options & cpu_to_be16(SFP_OPTIONS_HIGH_POWER_LEVEL))
 		power_mW = 2000;
 
+	supports_a2 = sfp->id.ext.sff8472_compliance !=
+				SFP_SFF8472_COMPLIANCE_NONE ||
+		      sfp->id.ext.diagmon & SFP_DIAGMON_DDM;
+
 	if (power_mW > sfp->max_power_mW) {
 		/* Module power specification exceeds the allowed maximum. */
-		if (sfp->id.ext.sff8472_compliance ==
-			SFP_SFF8472_COMPLIANCE_NONE &&
-		    !(sfp->id.ext.diagmon & SFP_DIAGMON_DDM)) {
+		if (!supports_a2) {
 			/* The module appears not to implement bus address
 			 * 0xa2, so assume that the module powers up in the
 			 * indicated mode.
@@ -1668,11 +1671,25 @@ static int sfp_module_parse_power(struct sfp *sfp)
 		}
 	}
 
+	if (power_mW <= 1000) {
+		/* Modules below 1W do not require a power change sequence */
+		sfp->module_power_mW = power_mW;
+		return 0;
+	}
+
+	if (!supports_a2) {
+		/* The module power level is below the host maximum and the
+		 * module appears not to implement bus address 0xa2, so assume
+		 * that the module powers up in the indicated mode.
+		 */
+		return 0;
+	}
+
 	/* If the module requires a higher power mode, but also requires
 	 * an address change sequence, warn the user that the module may
 	 * not be functional.
 	 */
-	if (sfp->id.ext.diagmon & SFP_DIAGMON_ADDRMODE && power_mW > 1000) {
+	if (sfp->id.ext.diagmon & SFP_DIAGMON_ADDRMODE) {
 		dev_warn(sfp->dev,
 			 "Address Change Sequence not supported but module requires %u.%uW, module may not be functional\n",
 			 power_mW / 1000, (power_mW / 100) % 10);