netdev - Re: [PATCH v7 06/11] net: pch_gbe: Only enable MAC when PHY link is active

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Date:   Wed, 27 Jun 2018 11:15:53 -0700
From:   Paul Burton <paul.burton@...s.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>,
        Andrew Lunn <andrew@...n.ch>
Subject: Re: [PATCH v7 06/11] net: pch_gbe: Only enable MAC when PHY link is
 active

Hi Florian,

On Wed, Jun 27, 2018 at 10:54:24AM -0700, Florian Fainelli wrote:
> On 06/26/2018 05:06 PM, Paul Burton wrote:
> > When using a PHY connected via RGMII, as the pch_gbe driver presumes is
> > the case, the RX clock is provided by the PHY to the MAC. Various PHYs,
> > including both the AR8031 used by the Minnowboard & the RTL8211E used by
> > the MIPS Boston development board, will stop generating the RX clock
> > when the ethernet link is down (eg. the ethernet cable is unplugged).
> > 
> > Various pieces of functionality in the EG20T MAC, ranging from basics
> > like completing a MAC reset to programming MAC addresses, rely upon the
> > RX clock being provided. When the clock is not provided these pieces of
> > functionality simply never complete, and the busy bits that indicate
> > they're in progress remain set indefinitely.
> > 
> > The pch_gbe driver currently requires that the RX clock is always
> > provided, and attempts to enforce this by disabling the hibernation
> > feature of the AR8031 PHY to keep it generating the RX clock. This patch
> > moves us away from this model by only configuring the MAC when the PHY
> > indicates that the ethernet link is up. When the link is up we should be
> > able to safely expect that the RX clock is being provided, and therefore
> > safely reset & configure the MAC.
> 
> What we ended up doing in the bcmgenet driver is loop back the RX and TX
> clocks such that we always have a clock that we can use to perform any
> MAC operation, including reset.
> 
> Is this an option here?

That sounds like a nice solution, but I don't see a way to do it in the
EG20T datasheet[1].

> You might also want to split the allocation from the actual
> initialization if this is not done already.

Some of the buffer allocation is still happening in pch_gbe_up() rather
than when the link actually comes up, but there is more that could
probably be moved.

Thanks,
    Paul

[1] https://www.intel.com/content/www/us/en/intelligent-systems/queens-bay/platform-controller-hub-eg20t-datasheet.html