linux-kernel - Re: [PATCH net-next v4 05/15] net: phy: Create a phy

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Message-ID: <20250217092911.772da5d0@fedora.home>
Date: Mon, 17 Feb 2025 09:29:11 +0100
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: davem@...emloft.net, netdev@...r.kernel.org,
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Subject: Re: [PATCH net-next v4 05/15] net: phy: Create a phy_port for
 PHY-driven SFPs

Hello Russell,

On Sat, 15 Feb 2025 18:57:01 +0000
"Russell King (Oracle)" <linux@...linux.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@...tlin.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 ?

Maxime