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

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Message-ID: <20250217154934.76ec03e5@fedora.home>
Date: Mon, 17 Feb 2025 15:49:34 +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

On Mon, 17 Feb 2025 14:21:29 +0000
"Russell King (Oracle)" <linux@...linux.org.uk> wrote:

> 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@...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 ?  
> 
> 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.

Yeah I'm thinking about moving only stuff that is phylink-independent
and only deals with linkmodes indeed. I'll spin a quick series to see
what it looks like then :)

Thanks,

Maxime