linux-kernel - Re: [PATCH] arm64: dts: qcom: sc7280-herobrine: Fix PCIe regulator glitch at bootup

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Message-ID: <Yip9EE+gcyRcSydd@google.com>
Date:   Thu, 10 Mar 2022 14:34:56 -0800
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Andy Gross <agross@...nel.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: dts: qcom: sc7280-herobrine: Fix PCIe regulator
 glitch at bootup

On Thu, Mar 10, 2022 at 01:04:34PM -0800, Douglas Anderson wrote:
> While scoping signals, we found that the PCIe signals weren't
> compliant at bootup. Specifically, the bootloader was setting up PCIe
> and leaving it configured, then jumping to the kernel. The kernel was
> turning off the regulator while leaving the PCIe clock running, which
> was a violation.
> 
> In the regulator bindings (and the Linux kernel driver that uses
> them), there's currently no way to specify that a GPIO-controlled
> regulator should keep its state at bootup. You've got to pick either
> "on" or "off". Let's switch it so that the PCIe regulator defaults to
> "on" instead of "off". This should be a much safer way to go and
> avoids the timing violation. The regulator will still be turned off
> later if there are no users.
> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>

Reviewed-by: Matthias Kaehlcke <mka@...omium.org>