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Date:   Sat, 7 Jan 2023 01:29:05 +0200
From:   Vladimir Oltean <olteanv@gmail.com>
To:     Sean Anderson <sean.anderson@seco.com>
Cc:     "Russell King (Oracle)" <linux@armlinux.org.uk>,
        Andrew Lunn <andrew@lunn.ch>,
        Heiner Kallweit <hkallweit1@gmail.com>, netdev@vger.kernel.org,
        "David S . Miller" <davem@davemloft.net>,
        Paolo Abeni <pabeni@redhat.com>, linux-kernel@vger.kernel.org,
        Jakub Kicinski <kuba@kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        Tim Harvey <tharvey@gateworks.com>
Subject: Re: [PATCH net-next v5 4/4] phy: aquantia: Determine rate adaptation
 support from registers
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On Fri, Jan 06, 2023 at 06:21:26PM -0500, Sean Anderson wrote:
> On 1/6/23 18:03, Vladimir Oltean wrote:
> > On Thu, Jan 05, 2023 at 05:46:48PM +0000, Russell King (Oracle) wrote:
> >> On Thu, Jan 05, 2023 at 07:34:45PM +0200, Vladimir Oltean wrote:
> >> > So we lose the advertisement of 5G and 2.5G, even if the firmware is
> >> > provisioned for them via 10GBASE-R rate adaptation, right? Because when
> >> > asked "What kind of rate matching is supported for 10GBASE-R?", the
> >> > Aquantia driver will respond "None".
> >> 
> >> The code doesn't have the ability to do any better right now - since
> >> we don't know what sets of interface modes _could_ be used by the PHY
> >> and whether each interface mode may result in rate adaption.
> >> 
> >> To achieve that would mean reworking yet again all the phylink
> >> validation from scratch, and probably reworking phylib and most of
> >> the PHY drivers too so that they provide a lot more information
> >> about their host interface behaviour.
> >> 
> >> I don't think there is an easy way to have a "perfect" solution
> >> immediately - it's going to take a while to evolve - and probably
> >> painfully evolve due to the slowness involved in updating all the
> >> drivers that make use of phylink in some way.
> > 
> > Serious question. What do we gain in practical terms with this patch set
> > applied? With certain firmware provisioning, some unsupported link modes
> > won't be advertised anymore. But also, with other firmware, some supported
> > link modes won't be advertised anymore.
> 
> Well, before the rate adaptation series, none of this would be
> advertised. I would rather add advertisement only for what we can
> actually support. We can always come back later and add additional
> support.

Well, yes. But practically, does it matter that we are negotiating a
link speed that we don't support, when the effect is the same (link
doesn't come up)? The only practical case I see is where advertising
e.g. an unsupported 2.5G would cause the link to not establish at a
supported 1G. But as you say, I don't think this will be the case with
the firmware provisioning that Tim gave as an example?

> > IIUC, Tim Harvey's firmware ultimately had incorrect provisioning, it's
> > not like the existing code prevents his use case from working.
> 
> The existing code isn't great as-is, since all the user sees is that we
> e.g. negotiated for 1G, but the link never came up.
> 
> --Sean