netdev - Re: [PATCH net-next 10/13] net: mvpp2: reset the XPCS while reconfiguring the serdes lanes

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Date:   Mon, 18 Feb 2019 11:26:30 +0100
From:   Antoine Tenart <antoine.tenart@...tlin.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Antoine Tenart <antoine.tenart@...tlin.com>, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        thomas.petazzoni@...tlin.com, maxime.chevallier@...tlin.com,
        gregory.clement@...tlin.com, miquel.raynal@...tlin.com,
        nadavh@...vell.com, stefanc@...vell.com, ymarkman@...vell.com,
        mw@...ihalf.com
Subject: Re: [PATCH net-next 10/13] net: mvpp2: reset the XPCS while
 reconfiguring the serdes lanes

Hi Russell,

On Fri, Feb 15, 2019 at 05:12:24PM +0000, Russell King - ARM Linux admin wrote:
> On Fri, Feb 15, 2019 at 04:32:38PM +0100, Antoine Tenart wrote:
> > The documentation advises to set the XPCS in reset while reconfiguring
> > the serdes lanes. This seems to be a good thing to do, but the PPv2
> > driver wasn't doing it. This patch fixes it.
> 
> Hmm.  That statment seems to have some ambiguity in it - we do two
> "reconfigurations" - one may be upon initialisation, where the lane
> is already configured for 10Gbase-KR, and we're re-initialising it
> for the same mode.  The other case is when we're switching between
> 10Gbase-KR and SGMII, or as will be the case with 2.5G support for
> the Alaska PHYs, 2500base-X.

The configuration at the lane at boot time is dependent to the
firmware or bootloader configuration. On the mcbin, the lane may be
configured in 10Gbase-KR, but it could be configured in SGMII as well.
The configuration upon initialization and the re-configuration are quite
similar then, as we might change mode as well at boot time.

You're right in that we might be re-configuring the lane for the same
exact mode at boot time, if it was already configured in the same mode.

> Does this apply to reconfiguration of the serdes lane between
> 10Gbase-KR and slower modes?

This applies only when configuring a line in a 10G mode,
mvpp22_gop_init_10gkr isn't called otherwise.

When switching to an non-10G mode we might want to put the XPCS in reset
though, which is not done today with this patch.

Thanks,
Antoine

-- 
Antoine Ténart, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com