linux-kernel - Re: [PATCH net-next v2 0/2] Enable 2.5Gbps speed for stmmac

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Date:   Tue, 6 Apr 2021 22:06:15 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     "Voon, Weifeng" <weifeng.voon@...el.com>
Cc:     "Sit, Michael Wei Hong" <michael.wei.hong.sit@...el.com>,
        "peppe.cavallaro@...com" <peppe.cavallaro@...com>,
        "alexandre.torgue@...com" <alexandre.torgue@...com>,
        "joabreu@...opsys.com" <joabreu@...opsys.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "mcoquelin.stm32@...il.com" <mcoquelin.stm32@...il.com>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>,
        "Ong, Boon Leong" <boon.leong.ong@...el.com>,
        "qiangqing.zhang@....com" <qiangqing.zhang@....com>,
        "Wong, Vee Khee" <vee.khee.wong@...el.com>,
        "fugang.duan@....com" <fugang.duan@....com>,
        "Chuah, Kim Tatt" <kim.tatt.chuah@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-stm32@...md-mailman.stormreply.com" 
        <linux-stm32@...md-mailman.stormreply.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hkallweit1@...il.com" <hkallweit1@...il.com>
Subject: Re: [PATCH net-next v2 0/2] Enable 2.5Gbps speed for stmmac

> The limitation is not on the MAC, PCS or the PHY. For Intel mgbe, the
> overclocking of 2.5 times clock rate to support 2.5G is only able to be
> configured in the BIOS during boot time. Kernel driver has no access to 
> modify the clock rate for 1Gbps/2.5G mode. The way to determined the 
> current 1G/2.5G mode is by reading a dedicated adhoc register through mdio bus.
> In short, after the system boot up, it is either in 1G mode or 2.5G mode 
> which not able to be changed on the fly. 

Right. It would of been a lot easier if this was in the commit message
from the beginning. Please ensure the next version does say this.

> Since the stmmac MAC can pair with any PCS and PHY, I still prefer that we tie
> this platform specific limitation with the of MAC. As stmmac does handle platform
> specific config/limitation. 

So yes, this needs to be somewhere in the intel specific stmmac code,
with a nice comment explaining what is going on.

What PHY are you using? The Aquantia/Marvell multi-gige phy can do
rate adaptation. So you could fix the MAC-PHY link to 2500BaseX, and
let the PHY internally handle the different line speeds.

    Andrew