netdev - Re: net: phylink: dsa: mv88e6xxx: flaky link detection on switch ports with internal PHYs

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Message-ID: <20190125164850.z7hkyr4f73q3enk5@e5254000004ec.dyn.armlinux.org.uk>
Date:   Fri, 25 Jan 2019 16:48:50 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     John David Anglin <dave.anglin@...l.net>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...oirfairelinux.com>,
        Florian Fainelli <f.fainelli@...il.com>, netdev@...r.kernel.org
Subject: Re: net: phylink: dsa: mv88e6xxx: flaky link detection on switch
 ports with internal PHYs

On Fri, Jan 25, 2019 at 11:30:54AM -0500, John David Anglin wrote:
> This also makes me wonder about this SD interrupt:
> 43:          0          0     GPIO1   3 Edge      d00d0000.sdhci cd
> Many poeple have trouble with SD cards on reboot.

[off topic, but a relevant reply to the above]

Very likely unrelated to edge interrupts.

For a signal that goes low when a card is inserted and high when
removed, you really need an edge interrupt to avoid flooding the CPU
with a "stuck" interrupt - if you were to use an active low interrupt
for card detection, and the signal is pulled low by card insertion,
the interrupt remains active until you remove the card!

In any case, I've seen the same with some early SolidRun platforms
when using high speed modes.

Most issues with SD cards at reboot is where the card is left in low
voltage/high speed mode, the reboot happens, and then you try to talk
to it in low-speed mode (as required by the specs for card detection/
initialisation) and they don't respond.  The only way out of that is
to reset the card - and the only way to reset the card is to remove
power from it.

If you're lucky enough to have a "regulator" which can be switched
off at reboot, that may not be sufficient - if there's a capacitor on
the SD card's supply line downstream from the regulator, the card can
be drawing soo little current that the voltage doesn't fall
sufficiently to cause the card to power down.

If you don't have any means to reset the card like that, then the only
workaround is to reduce the maximum speed - and avoid going to 1.8V
signalling mode.

-- 
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