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

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Message-ID: <535a0566-5c96-3477-9b94-332d54bf724e@seco.com>
Date:   Thu, 19 Jan 2023 13:32:59 -0500
From:   Sean Anderson <sean.anderson@...o.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     "Russell King (Oracle)" <linux@...linux.org.uk>,
        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 18:29, Vladimir Oltean wrote:
> 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?

I suppose.

I still think we should try to prevent bad firmware from tripping us up.
At the very least, I think we could detect bad configurations and warn
about them, so the user knows it's the firmware and not us.

--Sean

>> > 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