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

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Message-ID: <20210120205510.4642c92d@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 20 Jan 2021 20:55:10 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Florian Fainelli <f.fainelli@...il.com>,
        Marek Vasut <marex@...x.de>, Andrew Lunn <andrew@...n.ch>,
        Paul Barker <pbarker@...sulko.com>
Cc:     netdev@...r.kernel.org,
        Michael Grzeschik <m.grzeschik@...gutronix.de>
Subject: Re: [PATCH net-next V2] net: dsa: microchip: Adjust reset release
 timing to match reference reset circuit

On Wed, 20 Jan 2021 18:10:59 -0800 Florian Fainelli wrote:
> On 1/20/2021 5:51 PM, Marek Vasut wrote:
> > 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" ?  
> 
> If this is a bug fix for a commit that is not only in 'net-next', then
> yes, targeting 'net' is more appropriate:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/networking/netdev-FAQ.rst#n28

Yup, in that case applied this one and the port map fix to net.

Thanks everyone!