netdev - Re: [PATCH] net: phy: micrel: reconfigure the phy on resume

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Date:   Thu, 14 Jan 2021 08:30:18 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Claudiu.Beznea@...rochip.com, andrew@...n.ch, davem@...emloft.net,
        kuba@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: phy: micrel: reconfigure the phy on resume

On 13.01.2021 23:01, Russell King - ARM Linux admin wrote:
> On Wed, Jan 13, 2021 at 10:34:53PM +0100, Heiner Kallweit wrote:
>> On 13.01.2021 13:36, Claudiu.Beznea@...rochip.com wrote:
>>> On 13.01.2021 13:09, Heiner Kallweit wrote:
>>>> On 13.01.2021 10:29, Claudiu.Beznea@...rochip.com wrote:
>>>>> It could enter in this mode based on request for standby or suspend-to-mem:
>>>>> echo mem > /sys/power/state
>>>>> echo standby > /sys/power/state
> 
> This is a standard way to enter S2R - I've used it many times in the
> past on platforms that support it.
> 
>> I'm not a Linux PM expert, to me it seems your use case is somewhere in the
>> middle between s2r and hibernation. I *think* the assumption with s2r is
>> that one component shouldn't simply cut the power to another component,
>> and the kernel has no idea about it.
> 
> When entering S2R, power can (and probably will) be cut to all system
> components, certainly all components that do not support wakeup. If
> the system doesn't support WoL, then that will include the ethernet
> PHY.
> 
I'm with you if we talk about a driver's suspend callback cutting power
to the component it controls, or at least putting it to a power-saving
state. However a driver shouldn't have to expect that during S2R somebody
else cuts the power. If this would be the case, then I think we wouldn't
need separate resume and restore pm callbacks in general.

> When resuming, the responsibility is of the kernel and each driver's
> .resume function to ensure that the hardware state is restored. Only
> each device driver that knows the device itself can restore the state
> of that device. In the case of an ethernet PHY, that is phylib and
> its associated PHY driver.
> 
Also in phylib we have separate functions mdio_bus_phy_resume() and
mdio_bus_phy_restore(), with the first one not fully reconfiguring
the PHY.

> One has to be a tad careful with phylib and PHYs compared to their
> MAC devices in terms of the resume order - it has not been unheard
> of over the years for a MAC device to be resumed before its connected
> PHY has been.
> 
Right.