linux-kernel - Re: [PATCH v7 7/7] PCI: qcom: Add OPP support to scale performance state of power domain

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Message-ID: <20240227234549.GA251722@bhelgaas>
Date: Tue, 27 Feb 2024 17:45:49 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Krishna chaitanya chundru <quic_krichai@...cinc.com>
Cc: Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konrad.dybcio@...aro.org>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Johan Hovold <johan+linaro@...nel.org>,
	Brian Masney <bmasney@...hat.com>,
	Georgi Djakov <djakov@...nel.org>, linux-arm-msm@...r.kernel.org,
	linux-pci@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, vireshk@...nel.org,
	quic_vbadigan@...cinc.com, quic_skananth@...cinc.com,
	quic_nitegupt@...cinc.com, quic_parass@...cinc.com
Subject: Re: [PATCH v7 7/7] PCI: qcom: Add OPP support to scale performance
 state of power domain

On Tue, Feb 27, 2024 at 05:36:38PM -0600, Bjorn Helgaas wrote:
> On Fri, Feb 23, 2024 at 08:18:04PM +0530, Krishna chaitanya chundru wrote:
> > QCOM Resource Power Manager-hardened (RPMh) is a hardware block which
> > maintains hardware state of a regulator by performing max aggregation of
> > the requests made by all of the clients.

> > It is manadate to scale the performance state based up on the PCIe speed
> > link operates so that SoC can run under optimum power conditions.
> 
> It sounds like it's more power efficient, but not actually
> *mandatory*.  Maybe something like this?
> 
>   The SoC can be more power efficient if we scale the performance
>   state based on the aggregate PCIe link speed.

Actually, maybe it would be better to say "aggregate PCIe link
bandwidth", because we use "speed" elsewhere (PCIE_SPEED2MBS_ENC(),
etc) to refer specifically to the data rate independent of the width.

> > Add Operating Performance Points(OPP) support to vote for RPMh state based
> > upon the speed link is operating.
> 
> "... based on the link speed."

"... based on the aggregate link bandwidth."

> > In PCIe certain speeds like GEN1x2 & GEN2x1 or GEN3x2 & GEN4x1 use
> > same bw and frequency and thus the OPP entry, so use frequency based
> > search to reduce number of entries in the OPP table.
> 
> GEN1x2, GEN2x1, etc are not "speeds".  I would say:
> 
>   Different link configurations may share the same aggregate speed,
>   e.g., a 2.5 GT/s x2 link and a 5.0 GT/s x1 link have the same speed
>   and share the same OPP entry.

  Different link configurations may share the same aggregate
  bandwidth, e.g., a 2.5 GT/s x2 link and a 5.0 GT/s x1 link
  have the same bandwidth and share the same OPP entry.