netdev - Re: [PATCH net-next] net: phy: perform a PHY reset on resume

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Date:   Sat, 11 Dec 2021 15:57:54 +0100
From:   Francesco Dolcini <francesco.dolcini@...adex.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Francesco Dolcini <francesco.dolcini@...adex.com>,
        philippe.schenker@...adex.com, andrew@...n.ch,
        qiangqing.zhang@....com, davem@...emloft.net, festevam@...il.com,
        kuba@...nel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: phy: perform a PHY reset on resume

Hello,

On Sat, Dec 11, 2021 at 02:15:54PM +0000, Russell King (Oracle) wrote:
> I don't particularly like this - this impacts everyone who is using
> phylib at this point, whereas no reset was happening if the reset was
> already deasserted here.

I understand your concern, but I do not believe that this can create any
issue. The code should be able to handle the situation in which the PHY
is getting out of reset at that time.

> In the opening remarks to this series, it is stated:
> 
>   If a hardware-design is able to control power to the Ethernet PHY and
>   relying on software to do a reset, the PHY does no longer work after
>   resuming from suspend, given the PHY does need a hardware-reset.
> 
> This requirement is conditional on the hardware design, it isn't a
> universal requirement and won't apply everywhere. I think it needs to
> be described in firmware that this action is required. That said...
> 
> Please check the datasheet on the PHY regarding application of power and
> reset. You may find that the PHY datasheet requires the reset to be held
> active from power up until the clock input is stable - this could mean
> you need some other arrangement to assert the PHY reset signal after
> re-applying power sooner than would happen by the proposed point in the
> kernel.

I checked before sending this patch, the phy is a KSZ9131 [1] and
according to the power sequence timing  the reset should be toggled after
the power-up. No requirement on the clock or other signals.

The HW design is quite simple, we have a regulator controlling the PHY
power, and a GPIO controlling the reset. On suspend we remove the power
(FEC driver), on resume after enabling the power the PHY require a
reset, but nobody is doing it.

The issue here is that the phy regulator is handled by the FEC driver,
while the RESET_N GPIO can be controlled by both the FEC driver or the
phylib.
The initial proposal was to handle this into the FEC driver, but it was
not considered a good idea, therefore this new proposal.

One more comment about that, I do not believe that asserting the reset
in the suspend path is a good idea, in the general situation in which
the PHY is powered in suspend the power-consumption is likely to be
higher if the device is in reset compared to software power-down using
the BMCR register.

> universal requirement and won't apply everywhere. I think it needs to
> be described in firmware that this action is required. That said...
Are you thinking at a DTS property? Not sure to understand how do you
envision this. At the moment the regulator is not handled by the phylib,
and the property should be something like reset-on-resume, I guess ...

Francesco

[1] https://ww1.microchip.com/downloads/en/DeviceDoc/00002841C.pdf