linux-kernel - Re: [PATCH net-next v4 1/2] net: dsa: microchip: ksz8: Make flow control, speed, and duplex on CPU port configurable

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Message-ID: <20230521043841.GA22442@pengutronix.de>
Date:   Sun, 21 May 2023 06:38:41 +0200
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Woojung Huh <woojung.huh@...rochip.com>,
        Andrew Lunn <andrew@...n.ch>,
        Arun Ramadoss <arun.ramadoss@...rochip.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Simon Horman <simon.horman@...igine.com>,
        "Russell King (Oracle)" <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, kernel@...gutronix.de,
        netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        UNGLinuxDriver@...rochip.com,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net-next v4 1/2] net: dsa: microchip: ksz8: Make flow
 control, speed, and duplex on CPU port configurable

On Sat, May 20, 2023 at 06:17:08PM +0300, Vladimir Oltean wrote:
> On Sat, May 20, 2023 at 07:03:17AM +0200, Oleksij Rempel wrote:
> > On Fri, May 19, 2023 at 11:34:49PM +0300, Vladimir Oltean wrote:
> > > On Fri, May 19, 2023 at 08:50:15PM +0200, Oleksij Rempel wrote:
> > > > Thank you for your feedback. I see your point. 
> > > > 
> > > > We need to remember that the KSZ switch series has different types of
> > > > ports. Specifically, for the KSZ8 series, there's a unique port. This
> > > > port is unique because it's the only one that can be configured with
> > > > global registers, and it is only one supports tail tagging. This special
> > > > port is already referenced in the driver by "dev->cpu_port", so I continued
> > > > using it in my patch.
> > > 
> > > Ok, I understand, so for the KSZ8 family, the assumption about which
> > > port will use tail tagging is baked into the hardware.
> > > 
> > > > It is important to note that while this port has an xMII interface, it
> > > > is not the only port that could have an xMII interface. Therefore, using
> > > > "dev->info->internal_phy" may not be the best way to identify this port,
> > > > because there can be ports that are not global/cpu, have an xMII
> > > > interface, but don't have an internal PHY.
> > > 
> > > Right, but since we're talking about phylink, the goal is to identify
> > > the xMII ports, not the CPU ports... This is a particularly denatured
> > > case because the xMII port is global and is also the CPU port.
> > 
> > I see. Do you have any suggestions for a better or more suitable
> > implementation? I'm open to ideas.
> 
> Trying to answer here for both questions. In the RFC/RFT patch set I had
> posted, I introduced the concept of "wacky" registers, which are registers
> which should be per port (and are accessed as per-port by the driver),
> but because there is a single such port in the switch, the hardware
> design degenerated into moving them in the global area. Nonetheless,
> treating the xMII global registers as per-port makes it possible for the
> common driver to share more code between KSZ8 and others.
> 
> If you look at ksz9477_phylink_mac_link_up() - renamed to just
> ksz_phylink_mac_link_up() in my patch set - hard enough, you can see
> that it makes an attempt to generalize the "link up" procedure for all
> switch families, via these regs and fields. At the end of that regfield
> series, I theoretically converted KSZ8765/KSZ8794/KSZ8795 to reuse
> ksz9477_phylink_mac_link_up(). Theoretically because no one commented
> on whether the result still worked.
> 
> I think that regfields and that KSZ_WACKY_REG_FIELD_8() are an avenue
> worth exploring here.
> 

Looks good, I like the idea with "wacky" registers!

Would you prefer that I start working on adapting your patch set to the
KSZ8873? Or should I make a review to move forward the existing patch set?

Just a heads up, I don't have access to the KSZ87xx series switches, so
I won't be able to test the changes on these models.

Let me know what you think and how we should proceed.

Regards,
Oleksij
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