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

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Date:   Mon, 4 Feb 2019 19:23:21 +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 08:03:37AM -0800, Bjorn Andersson wrote:

> We have a regulator that is described as 1.05V in the schematics for the
> board we're working on and we have the USB block wanting 1.05V on one of
> its pins.  But the particular regulator works in steps of 8mV, and the
> adjacent steps are 1.048V and 1.056V.

> A) If we describe the min-microvolt = max-microvolt = 1.05V then the
> regulator frameworks adjusts the min value to 1.056V, per the steps, and
> fail as min > max.

Right, because that constraint isn't satisifiable on the board, an exact
value has been asked for which can't be delivered.  We *could* try to
allow some fudge factor for tolerance but that feels risky, it'd be
better if the constraints were written to be satisfiable.

> B) The USB driver that we inherited was written to request min/max of
> 1.05V/1.05V, so pointing this to a regulator with min/max of e.g.
> 1.05V/1.06V we're outside the adjusted range of 1.056V/1.06V.

This is just very bad practice on the part of the USB driver, if it is
not varying the voltage it should not be setting the voltage and let the
machine figure things out.  It is completely pointless for the driver to
be setting an exact value that it never varies, this is the sort of
thing machine constraints are for as it leads to trouble like this.
This is one of the many unfortunate practices in the Qualcomm BSP code
sadly.

> So the question is, should the board dts be written with
> min/max-microvolt adjusted to match the hardware steps? Or could the

Well, it is definitely unwise of the board to request a specific
voltage if the board can't physically set it, that's asking for trouble,
so I'd have expected the board should pick the value it wants.

> 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.

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