netdev - Re: [BUG] fec mdio times out under system stress

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Message-ID: <20190811164422.GF14290@lunn.ch>
Date:   Sun, 11 Aug 2019 18:44:22 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Fabio Estevam <festevam@...il.com>, netdev@...r.kernel.org,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>
Subject: Re: [BUG] fec mdio times out under system stress

> Maybe phylib should retry a number of times - but with read-sensitive
> registers, if the read has already completed successfully, and its
> just a problem with the FEC MDIO hardware, that could cause issues.

Hi Russell

At the bus level, MDIO cannot fail. The bits get clocked out, and the
bits get clocked in. There is no way for the PHY to stretch the clock
as I2C slaves can. There is nothing like the USB NACK, try again
later.

If something fails, it fails at a higher level, which means it is a
driver issue. In this case, the interrupt got delayed, after the timer
interrupt.

The FEC is also quite unusual in using an interrupt. Most MDIO drivers
poll. And if time gets 'stretched' because the system is too busy,
generally, the right thing happens anyway.

So i don't think it is phylib job to work around this issue.

   Andrew