Re: [PATCH net-next v4 05/15] net: phy: Create a phy_port for PHY-driven SFPs

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Mon, Feb 17, 2025 at 09:29:11AM +0100, Maxime Chevallier wrote:
> Hello Russell,
> 
> On Sat, 15 Feb 2025 18:57:01 +0000
> "Russell King (Oracle)" <linux@armlinux.org.uk> wrote:
> 
> > On Thu, Feb 13, 2025 at 11:15:53AM +0100, Maxime Chevallier wrote:
> > > Some PHY devices may be used as media-converters to drive SFP ports (for
> > > example, to allow using SFP when the SoC can only output RGMII). This is
> > > already supported to some extend by allowing PHY drivers to registers
> > > themselves as being SFP upstream.
> > > 
> > > However, the logic to drive the SFP can actually be split to a per-port
> > > control logic, allowing support for multi-port PHYs, or PHYs that can
> > > either drive SFPs or Copper.
> > > 
> > > To that extent, create a phy_port when registering an SFP bus onto a
> > > PHY. This port is considered a "serdes" port, in that it can feed data
> > > to anther entity on the link. The PHY driver needs to specify the
> > > various PHY_INTERFACE_MODE_XXX that this port supports.
> > > 
> > > Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>  
> > 
> > With this change, using phy_port requires phylink to also be built in
> > an appropriate manner. Currently, phylink depends on phylib. phy_port
> > becomes part of phylib. This patch makes phylib depend on phylink,
> > thereby creating a circular dependency when modular.
> > 
> > I think a different approach is needed here.
> 
> That's true.
> 
> One way to avoid that would be to extract out of phylink/phylib all the
> functions for linkmode handling that aren't tied to phylink/phylib
> directly, but are about managing the capabilities of each interface,
> linkmode, speed, duplex, etc. For phylink, that would be :
> 
> phylink_merge_link_mode
> phylink_get_capabilities
> phylink_cap_from_speed_duplex
> phylink_limit_mac_speed
> phylink_caps_to_linkmodes
> phylink_interface_max_speed
> phylink_interface_signal_rate
> phylink_is_empty_linkmode
> phylink_an_mode_str
> phylink_set_port_modes
> 
> For now all these are phylink internal and that makes sense, but if we want
> phy-driven SFP support, stackable PHYs and so on, we'll need some ways for
> the PHY to expose its media-side capabilities, and we'd reuse these.
> 
> These would go into linkmode.c/h for example, and we'd have a shared set
> of helpers that we can use in phylink, phylib and phy_port.
> 
> Before I go around and rearrange that, are you OK with this approach ?

I'm not convinced. If you're thinking of that level of re-use, you're
probably going to miss out on a lot of logic that's in phylink. Maybe
there should be a way to re-use phylink in its entirety between the
PHY and SFP.

Some of the above (that deal only with linkmodes) would make sense
to move out though.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!