netdev - Re: [PATCH RFC v1 0/7] net: phy: Ethernet PHY powerdown optimization

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Message-Id: <20131120.153642.2045230991917379052.davem@davemloft.net>
Date:	Wed, 20 Nov 2013 15:36:42 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	sebastian.hesselbarth@...il.com
Cc:	netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v1 0/7] net: phy: Ethernet PHY powerdown
 optimization

From: Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
Date: Wed, 20 Nov 2013 21:21:46 +0100

> Ethernet PHYs consume a significant amount of power when link is detected.
> Especially, for embedded systems it can be easily 20-40% of total system
> power. Now, currently most likely all ethernet drivers leave PHYs powered
> on, even if the device is taken down. Also, some stupid boot loaders power
> on all PHYs available.
> 
> This RFC deals with saving power consumed by ethernet PHYs, that have no
> corresponding ethernet device driver or are attached to ethernet devices
> which are taken down by user request, i.e. ifconfig ethN down. Ports with
> no link, i.e. cable removed, are already quite good at power saving due to
> PHY internal link detection.

The idea is sound and the goal is of course valuable, but it brings up
a chronically reoccurring issue as of late.

You cannot reset the PHY or take it down without somehow retaining the
settings the PHY had when you bring it back up.

If I ifdown/ifup a device, my ethtool link configuration better be
retained.

This means the PHY layer must have a way to reprogram the device when
it is brought back up, with whatever settings the software state
things are there.
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