linux-kernel - Re: [PATCH v3 09/14] arm64: dts: qcom: sc7280: Disable pull from pcie1

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Date:   Thu, 3 Feb 2022 21:42:05 +0000
From:   Stephen Boyd <swboyd@...omium.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Douglas Anderson <dianders@...omium.org>
Cc:     pmaliset@...eaurora.org, mka@...omium.org,
        quic_rjendra@...cinc.com,
        Shaik Sajida Bhanu <sbhanu@...eaurora.org>,
        kgodara@...eaurora.org, konrad.dybcio@...ainline.org,
        Sankeerth Billakanti <quic_sbillaka@...cinc.com>,
        sibis@...eaurora.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 v3 09/14] arm64: dts: qcom: sc7280: Disable pull from pcie1_clkreq

Quoting Douglas Anderson (2022-02-02 13:23:43)
> I believe that the PCIe clkreq pin is an output. That means we
> shouldn't have a pull enabled for it. Turn it off.

It sounds like it's a request from the PCI device to the PCI phy that
the clk should be on. I googled pcie clkreq open drain and this pdf[1]
says

"The CLKREQ# signal is an open drain, active low signal that is driven
low by the PCI Express M.2 add-I Card function to request that the PCI
Express reference clock be available (active clock state) in order to
allow the PCI Express interface to send/receive data"

so presumably if there isn't an external pull on the signal the open
drain feature will not work and the PCIe device won't be able to drive
it low.

[1] https://advdownload.advantech.com/productfile/PIS/96FD80-P512-LIS/Product%20-%20Datasheet/96FD80-P512-LIS_datasheet20180110154919.pdf