linux-kernel - Re: [PATCH 2/2] PCI: qcom: Add basic interconnect support

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Date:   Wed, 19 Oct 2022 16:45:09 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Brian Masney <bmasney@...hat.com>
Cc:     Johan Hovold <johan+linaro@...nel.org>,
        Stanimir Varbanov <svarbanov@...sol.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Manivannan Sadhasivam <mani@...nel.org>,
        Krishna chaitanya chundru <quic_krichai@...cinc.com>,
        quic_vbadigan@...cinc.com, linux-arm-msm@...r.kernel.org,
        linux-pci@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] PCI: qcom: Add basic interconnect support

On Wed, Oct 19, 2022 at 10:32:41AM -0400, Brian Masney wrote:
> On Mon, Oct 17, 2022 at 01:24:49PM +0200, Johan Hovold wrote:
> > +	/*
> > +	 * Some Qualcomm platforms require interconnect bandwidth constraints
> > +	 * to be set before enabling interconnect clocks.
> > +	 *
> > +	 * Set an initial peak bandwidth corresponding to single-lane Gen 1
> > +	 * for the pcie-mem path.
> > +	 */
> > +	ret = icc_set_bw(pcie->icc_mem, 0, MBps_to_icc(250));
> 
> [snip]
> 
> > +	speed = FIELD_GET(PCI_EXP_LNKSTA_CLS, status);
> > +	width = FIELD_GET(PCI_EXP_LNKSTA_NLW, status);
> > +
> > +	switch (speed) {
> > +	case 1:
> > +		bw = MBps_to_icc(250);
> > +		break;
> > +	case 2:
> > +		bw = MBps_to_icc(500);
> > +		break;
> > +	default:
> > +	case 3:
> > +		bw = MBps_to_icc(985);
> > +		break;
> > +	}
> 
> Just curious: These platforms have a 4 lane PCIe bus. Why use 985
> instead of 1000 for the maximum?

This is the per-lane bandwidth that depends on encoding. The four-lane
peak throughput would be about 3.94 GB/s.

Johan