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

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Date:   Thu, 3 Feb 2022 13:53:09 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Prasad Malisetty <pmaliset@...eaurora.org>,
        Matthias Kaehlcke <mka@...omium.org>, quic_rjendra@...cinc.com,
        Shaik Sajida Bhanu <sbhanu@...eaurora.org>,
        kgodara@...eaurora.org,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Sankeerth Billakanti <quic_sbillaka@...cinc.com>,
        Sibi Sankar <sibis@...eaurora.org>,
        Andy Gross <agross@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 09/14] arm64: dts: qcom: sc7280: Disable pull from pcie1_clkreq

Hi,

On Thu, Feb 3, 2022 at 1:42 PM Stephen Boyd <swboyd@...omium.org> wrote:
>
> 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

Yeah, I had some trouble figuring this out too, so if someone knows
better than me then I'm more than happy to take advice here. I thought
I had found something claiming that "clkreq" was an output and on the
schematic I have from Qualcomm it shows an arrow going out from the
SoC for this signal indicating that it's an output from the SoC. Of
course, those arrows are notoriously wrong but at least it's one piece
of evidence that someone thought this was an output from the SoC.

Hrm, but I just checked the sc7280 "datasheet" which claims that this
is an input. Sigh.

I guess the options are:
* If we're sure this is an input to the SoC then I think we should
remove the drive-strength, right?
* If we don't know then I guess we can leave both?


In any case, for now we can just drop this patch?

-Doug