linux-kernel - Re: [PATCH net-next v5 4/4] phy: aquantia: Determine rate adaptation support from registers

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Date:   Fri, 6 Jan 2023 18:21:26 -0500
From:   Sean Anderson <sean.anderson@...o.com>
To:     Vladimir Oltean <olteanv@...il.com>,
        "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>, netdev@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>, linux-kernel@...r.kernel.org,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Tim Harvey <tharvey@...eworks.com>
Subject: Re: [PATCH net-next v5 4/4] phy: aquantia: Determine rate adaptation
 support from registers

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.

> 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