netdev - Re: [PATCH net-next V2] net: dsa: microchip: Adjust reset release timing to match reference reset circuit

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Date:   Thu, 21 Jan 2021 02:51:21 +0100
From:   Marek Vasut <marex@...x.de>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Michael Grzeschik <m.grzeschik@...gutronix.de>,
        Paul Barker <pbarker@...sulko.com>
Subject: Re: [PATCH net-next V2] net: dsa: microchip: Adjust reset release
 timing to match reference reset circuit

On 1/21/21 2:31 AM, Jakub Kicinski wrote:
> On Wed, 20 Jan 2021 04:05:02 +0100 Marek Vasut wrote:
>> KSZ8794CNX datasheet section 8.0 RESET CIRCUIT describes recommended
>> circuit for interfacing with CPU/FPGA reset consisting of 10k pullup
>> resistor and 10uF capacitor to ground. This circuit takes ~100 ms to
>> rise enough to release the reset.
>>
>> For maximum supply voltage VDDIO=3.3V VIH=2.0V R=10kR C=10uF that is
>>                      VDDIO - VIH
>>    t = R * C * -ln( ------------- ) = 10000*0.00001*-(-0.93)=0.093 s
>>                         VDDIO
>> so we need ~95 ms for the reset to really de-assert, and then the
>> original 100us for the switch itself to come out of reset. Simply
>> msleep() for 100 ms which fits the constraint with a bit of extra
>> space.
>>
>> Fixes: 5b797980908a ("net: dsa: microchip: Implement recommended reset timing")
>> Reviewed-by: Florian Fainelli <f.fainelli@...il.com>
>> Signed-off-by: Marek Vasut <marex@...x.de>
> 
> I'm slightly confused whether this is just future proofing or you
> actually have a board where this matters. The tree is tagged as
> net-next but there is a Fixes tag which normally indicates net+stable.

I have a board where I trigger this problem, that's how I found it. It 
should be passed to stable too. So the correct tree / tag is "net" ?