netdev - Re: [PATCH] net: phy: at803x: Improve hibernation support on start up

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Message-ID: <20230809060836.GA13300@pengutronix.de>
Date: Wed, 9 Aug 2023 08:08:36 +0200
From: Oleksij Rempel <o.rempel@...gutronix.de>
To: Wei Fang <wei.fang@....com>
Cc: Marek Vasut <marex@...x.de>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Andrew Lunn <andrew@...n.ch>, Eric Dumazet <edumazet@...gle.com>,
	Heiner Kallweit <hkallweit1@...il.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Russell King <linux@...linux.org.uk>, kernel@...gutronix.de,
	linux-clk@...r.kernel.org, Stephen Boyd <sboyd@...nel.org>,
	Michael Turquette <mturquette@...libre.com>
Subject: Re: [PATCH] net: phy: at803x: Improve hibernation support on start up

CCing clk folks in the loop.

On Wed, Aug 09, 2023 at 05:37:45AM +0000, Wei Fang wrote:
...
> > Hm.. how about officially defining this PHY as the clock provider and disable
> > PHY automatic hibernation as long as clock is acquired?
> > 
> Sorry, I don't know much about the clock provider/consumer, but I think there
> will be more changes if we use clock provider/consume mechanism.

Yes, more changes will be needed.

> Furthermore,
> we would expect the hibernation mode is enabled when the ethernet interface
> is brought up but the cable is not plugged, that is to say, we only need the PHY to
> provide the clock for a while to make the MAC reset successfully. 

Means, if external clock is not provided, MAC is not fully functional.
Correct?

What kind of MAC operation will fail in this case?
For example, if stmmac_open() fails without external clock, will
stmmac_release() work properly? Will we be able to do any configuration
on an interface which is opened, but without active link and hibernated
clock? How about self tests?

> Therefore, I think
> the current approach is more simple and effective, and it takes full advantage of the
> characteristics of the hardware (The PHY will continue to provide the clock about
> 10 seconds after hibernation mode is enabled when the cable is not plugged and
> automatically disable the clock after 10 seconds, so the 10 seconds is enough for
> the MAC to reset successfully).

If multiple independent operations are synchronized based on the
assumption that 10 seconds should be enough, bad thing happens.

If fully functional external clock provider is need to initialize the
MAC, just disabling this clock on already initialized HW without doing
proper re-initialization sequence is usually bad idea. HW may get some
glitch which will make troubleshooting a pain.

Regards,
Oleksij
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