Date:   Fri, 31 May 2019 18:33:32 -0600
From:   Robert Hancock <hancock@...systems.ca>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: phy: phylink: support using device PHY in
 fixed or 802.3z mode

On 2019-05-31 2:31 p.m., Russell King - ARM Linux admin wrote:
> On Fri, May 31, 2019 at 01:18:05PM -0600, Robert Hancock wrote:
>> The Xilinx AXI Ethernet controller supports SFP modules in 1000BaseX
>> mode in a somewhat unusual manner: it still exposes a PHY device which
>> needs some PHY-level initialization for the PCS/PMA layer to work properly,
>> and which provides some link status/control information.
>>
>> In this case, we want to use the phylink layer to support proper
>> communication with the SFP module, but in most other respects we want to
>> use the PHY attached to the controller.
>>
>> Currently the phylink driver does not initialize or use a controller PHY
>> even if it exists for fixed-link or 802.3z PHY modes, and doesn't
>> support SFP module attachment in those modes.
> 
> Sorry, I'm having a hard time following this description.  Please draw
> an ASCII diagram of the setup you have - a picture is worth 1000 words,
> and I think that is very much the case here.
>
> We do have boards where the SFP is connected to a real PHY, where the
> real PHY offers a RJ45 copper socket and a fiber interface,
> automatically switching between the two.  In this case, we do not
> use phylink to represent the link between the PHY and the SFP cage,
> but instead the PHY binds directly to the SFP cage.
>

It sounds like the setup you're describing has the PHY being smarter and
doing more of the SFP module handling internally. In our setup, the
situation is something like this:

Xilinx MAC		I2C	GPIO
|
|GMII			|	|
|			|	|
Xilinx PHY		|	|
|			|	|
|1000BaseX		|	|
|			|	|
SFP -----------------------------

So in this case the Xilinx PHY is just really doing PCS/PMA, etc.
conversions. The I2C and GPIO lines for the SFP socket are routed back
to the host separately and the Xilinx PHY has no interaction with them
(other than that I believe the LOS signal from the SFP socket is
connected into the PHY to provide some link status indication back to it).

In this setup, phylink is basically allowing us to communicate with the
SFP module over I2C and manage the LOS, TX enable, etc. control lines
properly, but the Xilinx PHY does need to be initialized as well for the
actual link traffic to pass through.

>> This change allows it to
>> utilize a controller PHY if it is defined, and allows SFP module
>> attachment/initialization but does not connect the PHY device to the
>> controller (to allow the controller PHY to be used for link state
>> tracking).
>>
>> Fully supporting this setup would probably require initializing and
>> tracking the state of both PHYs, which is a much more complex change and
>> doesn't appear to be required for this use case.
>>
>> Signed-off-by: Robert Hancock <hancock@...systems.ca>
>> ---
>>  drivers/net/phy/phylink.c | 29 +++++++++++++++++++----------
>>  1 file changed, 19 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/net/phy/phylink.c b/drivers/net/phy/phylink.c
>> index 4fd72c2..9362aca 100644
>> --- a/drivers/net/phy/phylink.c
>> +++ b/drivers/net/phy/phylink.c
>> @@ -819,12 +819,6 @@ int phylink_of_phy_connect(struct phylink *pl, struct device_node *dn,
>>  	struct phy_device *phy_dev;
>>  	int ret;
>>  
>> -	/* Fixed links and 802.3z are handled without needing a PHY */
>> -	if (pl->link_an_mode == MLO_AN_FIXED ||
>> -	    (pl->link_an_mode == MLO_AN_INBAND &&
>> -	     phy_interface_mode_is_8023z(pl->link_interface)))
>> -		return 0;
>> -
> 
> This looks to me like it will break existing users.
> 
>>  	phy_node = of_parse_phandle(dn, "phy-handle", 0);
>>  	if (!phy_node)
>>  		phy_node = of_parse_phandle(dn, "phy", 0);
>> @@ -1697,9 +1691,6 @@ static int phylink_sfp_module_insert(void *upstream,
>>  		    phy_modes(config.interface),
>>  		    __ETHTOOL_LINK_MODE_MASK_NBITS, support);
>>  
>> -	if (phy_interface_mode_is_8023z(iface) && pl->phydev)
>> -		return -EINVAL;
>> -
>>  	changed = !bitmap_equal(pl->supported, support,
>>  				__ETHTOOL_LINK_MODE_MASK_NBITS);
>>  	if (changed) {
>> @@ -1751,12 +1742,30 @@ static int phylink_sfp_connect_phy(void *upstream, struct phy_device *phy)
>>  {
>>  	struct phylink *pl = upstream;
>>  
>> +	/* In fixed mode, or in in-band mode with 802.3z PHY interface mode,
>> +	 *  ignore the SFP PHY and just use the PHY attached to the MAC.
>> +	 */
>> +	if (pl->link_an_mode == MLO_AN_FIXED ||
>> +	    (pl->link_an_mode == MLO_AN_INBAND &&
>> +	      phy_interface_mode_is_8023z(pl->link_config.interface)))
>> +		return 0;
>> +
>>  	return __phylink_connect_phy(upstream, phy, pl->link_config.interface);
>>  }
>>  
>>  static void phylink_sfp_disconnect_phy(void *upstream)
>>  {
>> -	phylink_disconnect_phy(upstream);
>> +	struct phylink *pl = upstream;
>> +
>> +	/* In fixed mode, or in in-band mode with 802.3z PHY interface mode,
>> +	 * ignore the SFP PHY and just use the PHY attached to the MAC.
>> +	 */
>> +	if (pl->link_an_mode == MLO_AN_FIXED ||
>> +	    (pl->link_an_mode == MLO_AN_INBAND &&
>> +	      phy_interface_mode_is_8023z(pl->link_config.interface)))
>> +		return;
> 
> Fixed link mode is mutually exclusive with there being a PHY present.
> Please see Documentation/devicetree/bindings/net/fixed-link.txt
> 
> Fixed links are not used to fix a declared PHY to a specific mode.

I agree it would likely make sense to not include fixed mode in this case.

> 
>> +
>> +	phylink_disconnect_phy(pl);
>>  }
>>  
>>  static const struct sfp_upstream_ops sfp_phylink_ops = {
> 
> Overall, I think you need to better describe what your setup is, and
> what you are trying to achieve:
> 
> * Are you merely trying to support copper SFP modules where the PHY
>   defaults to 1000base-X mode rather than SGMII?
> * Are you trying to support a network controller that doesn't support
>   SGMII mode?

In our case the controller is supporting 1000BaseX only and is mainly
intended for fiber modules. We do want to be able to get copper modules
to work - obviously only ones that are set up for 1000BaseX mode are
possible.

> 
> If the former, then I'm pretty certain you're going about it the wrong
> way - as I've said before, there is nothing in the EEPROM that
> indicates definitively what format the control word is (and therefore
> whether it is SGMII or 1000base-X.)
> 
> Some network controllers may be able to tell the difference, but that
> is not true of all controllers.
> 
> The only way I can see to support such modules would be to have a table
> of quirks to set the interface mode accordingly, and not try this "lets
> pick one, try to validate it with the network controller, otherwise try
> the other."
> 
> In any case, the format of the connection between the SFP module and
> the network controller isn't one that should appear in the ethtool link
> modes - I view what you've done there as a hack rather than proper
> design.
> 
> If, on the other hand it is the latter, what you do you expect to
> happen if you plug a copper SFP module that only supports SGMII into
> a network controller that only supports 1000baseX ?  The PHY on some
> of these modules won't pass data unless the SGMII handshake with the
> network controller completes, which it may or may not do depending on
> the 1000baseX implementation - but the network controller won't
> interpret the bits correctly, and certainly won't be able to deal
> with it when the link switches to 100M or 10M mode, which it will do
> depending on the results of the copper side negotiation.
> 

-- 
Robert Hancock
Senior Software Developer
SED Systems, a division of Calian Ltd.
Email: hancock@...systems.ca

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