Subject: Re: [PATCH net,stable] phy: sfp: fix high power modules without diag mode

From: "Russell King (Oracle)" <[email-masked]>
Date: Fri, 3 Dec 2021 12:58:03 +0000

On Fri, Dec 03, 2021 at 11:54:57AM +0000, Russell King (Oracle) wrote:
> On Thu, Dec 02, 2021 at 05:58:43PM -0800, Jakub Kicinski wrote:
> > On Tue, 30 Nov 2021 08:39:29 +0100 Bjørn Mork wrote:
> > > Commit 7cfa9c92d0a3 ("net: sfp: avoid power switch on address-change
> > > modules") changed semantics for high power modules without diag mode.
> > > We repeatedly try to read the current power status from the non-existing
> > > 0xa2 address, in the 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
> > > 
> > > Eeprom dump:
> > > 
> > > 0x0000: 03 04 01 00 00 00 80 00 00 00 00 01 0d 00 0a 64
> > > 0x0010: 00 00 00 00 4e 54 54 20 20 20 20 20 20 20 20 20
> > > 0x0020: 20 20 20 20 00 00 00 00 30 30 30 30 30 30 30 30
> > > 0x0030: 30 30 30 30 30 30 30 30 30 30 30 30 05 1e 00 7d
> > > 0x0040: 02 00 00 00 30 30 30 30 30 30 30 30 30 30 30 30
> > > 0x0050: 30 30 30 30 31 36 30 34 30 38 20 20 00 00 00 75
> > > 0x0060: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > 0x0070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > 0x0080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > 0x0090: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > 0x00a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > 0x00b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > 0x00c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > 0x00d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > 0x00e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > 0x00f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > 
> > > Previously we 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.
> > > 
> > > Revert to that behaviour, refactoring to keep the improved
> > > diagnostic messages.
> > > 
> > > Fixes: 7cfa9c92d0a3 ("net: sfp: avoid power switch on address-change modules")
> > > Reported-and-tested-by: 照山周一郎 <[email-masked]>
> > > Cc: Russell King <[email-masked]>
> > > Signed-off-by: Bjørn Mork <[email-masked]>
> > 
> > Russell, any comments?
> 
> Sorry for the delay, I've been out over the last couple of days. I 
> hink it's fine, but the code here is not easy to understand, hence
> why this subtlety was missed. So, I'm not entirely happy about going
> back to the original code.
> 
> Maybe instead doing a check in sfp_sm_mod_hpower() for this would
> be better? Possibly something like:
> 
> 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) {
> ...
> 	}
> +
> +	if (!supports_a2 && power_mW > 1000) {
> +		/* 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
> ...
> 
> This way, if the module reports it doesn't support 0xa2, we don't get
> the "Address Change Sequence not supported" message - since if 0xa2 is
> not supported, then the address change sequence is irrelevant. However,
> modules shouldn't have that bit set... but "shouldn't" doesn't mean
> they do not.
> 
> This also has the advantage of making the check explicit and obvious,
> and I much prefer the organisation of:
> 
> 	if (module_exceeds_host_power) {
> 		handle this case
> 	} else {
> 		do other checks
> 	}
> 
> I think maybe dealing with power_mW <= 1000 early on may be a good idea,
> and eliminates the tests further down for power_mW > 1000.
> 
> 	if (power_mW <= 1000) {
> 		sfp->module_power_mW = power_mW;
> 		return 0;
> 	}
> 
> since those modules do not require any special handling.

Thinking a little more, how about this:

 drivers/net/phy/sfp.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/drivers/net/phy/sfp.c b/drivers/net/phy/sfp.c
index 51a1da50c608..4c900d063b19 100644
--- a/drivers/net/phy/sfp.c
+++ b/drivers/net/phy/sfp.c
@@ -1752,17 +1752,20 @@ 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.
@@ -1779,11 +1782,24 @@ static int sfp_module_parse_power(struct sfp *sfp)
 		}
 	}
 
+	if (power_mW <= 1000) {
+		/* Modules below 1W do not require a power change sequence */
+		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);

-- 
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