linux-kernel - Re: [PATCH v2 1/8] dt-bindings: PCI: Add binding for qps615

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Message-ID: <20240822140956.unt45fgpleqwniwa@thinkpad>
Date: Thu, 22 Aug 2024 19:39:56 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Krzysztof Kozlowski <krzk@...nel.org>
Cc: Bjorn Andersson <quic_bjorande@...cinc.com>,
	Krishna Chaitanya Chundru <quic_krichai@...cinc.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Konrad Dybcio <konrad.dybcio@...aro.org>,
	cros-qcom-dts-watchers@...omium.org,
	Bartosz Golaszewski <brgl@...ev.pl>,
	Jingoo Han <jingoohan1@...il.com>, andersson@...nel.org,
	quic_vbadigan@...cinc.com, linux-arm-msm@...r.kernel.org,
	linux-pci@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v2 1/8] dt-bindings: PCI: Add binding for qps615

On Thu, Aug 08, 2024 at 03:06:28PM +0200, Krzysztof Kozlowski wrote:
> On 08/08/2024 14:41, Manivannan Sadhasivam wrote:
> > On Thu, Aug 08, 2024 at 02:13:01PM +0200, Krzysztof Kozlowski wrote:
> >> On 08/08/2024 14:01, Manivannan Sadhasivam wrote:
> >>> On Mon, Aug 05, 2024 at 07:18:04PM +0200, Krzysztof Kozlowski wrote:
> >>>> On 05/08/2024 19:07, Bjorn Andersson wrote:
> >>>>> On Mon, Aug 05, 2024 at 09:41:26AM +0530, Krishna Chaitanya Chundru wrote:
> >>>>>> On 8/4/2024 2:23 PM, Krzysztof Kozlowski wrote:
> >>>>>>> On 03/08/2024 05:22, Krishna chaitanya chundru wrote:
> >>>>>>>> diff --git a/Documentation/devicetree/bindings/pci/qcom,qps615.yaml b/Documentation/devicetree/bindings/pci/qcom,qps615.yaml
> >>>>> [..]
> >>>>>>>> +  qps615,axi-clk-freq-hz:
> >>>>>>>> +    description:
> >>>>>>>> +      AXI clock which internal bus of the switch.
> >>>>>>>
> >>>>>>> No need, use CCF.
> >>>>>>>
> >>>>>> ack
> >>>>>
> >>>>> This is a clock that's internal to the QPS615, so there's no clock
> >>>>> controller involved and hence I don't think CCF is applicable.
> >>>>
> >>>> AXI does not sound that internal.
> >>>
> >>> Well, AXI is applicable to whatever entity that implements it. We mostly seen it
> >>> in ARM SoCs (host), but in this case the PCIe switch also has a microcontroller
> >>> /processor of some sort, so AXI is indeed relevant for it. The naming actually
> >>> comes from the switch's i2c register name that is being configured in the driver
> >>> based on this property value.
> >>>
> >>>> DT rarely needs to specify internal
> >>>> clock rates. What if you want to define rates for 20 clocks? Even
> >>>> clock-frequency is deprecated, so why this would be allowed?
> >>>> bus-frequency is allowed for buses, but that's not the case here, I guess?
> >>>>
> >>>
> >>> This clock frequency is for the switch's internal AXI bus that runs at default
> >>> 200MHz. And this property is used to specify a frequency that is configured over
> >>> the i2c interface so that the switch's AXI bus can operate in a low frequency
> >>> there by reducing the power consumption of the switch.
> >>>
> >>> It is not strictly needed for the switch operation, but for power optimization.
> >>> So this property can also be dropped for the initial submission and added later
> >>> if you prefer.
> >>
> >> So if the clock rate can change, why this is static in DTB? Or why this
> >> is configurable per-board?
> >>
> > 
> > Because, board manufacturers can change the frequency depending on the switch
> > configuration (enablement of DSP's etc...)
> > 
> >> There is a reason why clock-frequency property is not welcomed and you
> >> are re-implementing it.
> >>
> > 
> > Hmm, I'm not aware that 'clock-frequency' is not encouraged these days. So you
> > are suggesting to change the rate in the driver itself based on the switch
> > configuration? If so, what difference does it make?
> 
> Based on the switch, other clocks, votes etc. whatever is reasonable
> there. In most cases, not sure if this one here as well, devices can
> operate on different clock frequencies thus specifying fixed frequency
> in the DTS is simplification and lack of flexibility. It is chosen by
> people only because it is easier for them but then they come back with
> ABI issues when it turns out they need to switch to some dynamic control.
> 

Atleast for this device, this frequency is going to be static. Because, the
device itself cannot change the frequency, only the host driver can. That too is
only possible before enumerating the device. So there is no way the frequency is
going to change dynamically.

- Mani

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