linux-kernel - Re: [PATCH] arm64: dts: qcs404: evb: Fix voltages for s5 and l3

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Message-ID: <20190204205930.GK23441@sirena.org.uk>
Date:   Mon, 4 Feb 2019 21:59:30 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Niklas Cassel <niklas.cassel@...aro.org>,
        Jorge Ramirez <jorge.ramirez-ortiz@...aro.org>,
        Andy Gross <andy.gross@...aro.org>,
        David Brown <david.brown@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Khasim Syed Mohammed <khasim.mohammed@...aro.org>
Subject: Re: [PATCH] arm64: dts: qcs404: evb: Fix voltages for s5 and l3

On Mon, Feb 04, 2019 at 11:25:10AM -0800, Bjorn Andersson wrote:
> On Mon 04 Feb 10:23 PST 2019, Mark Brown wrote:
> > On Mon, Feb 04, 2019 at 08:03:37AM -0800, Bjorn Andersson wrote:

> Okay, I don't see a big problem with expecting the DT to only contain
> values that the hardware can actually do. I do however see that
> machine_constraints_voltage() has been changes to expect the driver to
> return -ENOTRECOVERABLE when we don't have a way to query the voltage
> during probe.

> So there's a risk of this breaking compatibility with older dtb files on
> other boards. We will have to do some more verification on this.

That's basically only going to affect Qualcomm hardware FWIW, I had
thought that such devices didn't manage to probe with upstream before
that change either but I could be misremembering.

> > > regulator framework be made to round down to the previous valid step
> > > instead of up?

> > We definitely don't want to round voltages down, it is vastly more
> > common for devices to experience problems like brownouts if they go
> > under voltage so it'd be more likely to cause harm than good.

> We're 2mV off in this case, but it could have been way off. So I'm good
> with this position.

Never mind that that's 2mV assuming the regulator has perfect accuracy
and there's headroom.  For something like this it's likely not a problem
but some high current things (especially CPUs or anything like that) are
operating on thin margins.

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