linux-kernel - Re: [PATCH v6 5/6] arm64: dts: qcom: sm8450: Add opp table support to PCIe

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Date: Thu, 1 Feb 2024 15:45:17 +0100
From: Konrad Dybcio <konrad.dybcio@...aro.org>
To: Viresh Kumar <viresh.kumar@...aro.org>,
 Manivannan Sadhasivam <mani@...nel.org>
Cc: Krishna chaitanya chundru <quic_krichai@...cinc.com>,
 Bjorn Andersson <andersson@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
 Lorenzo Pieralisi <lpieralisi@...nel.org>,
 Krzysztof Wilczyński <kw@...ux.com>,
 Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.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,
 vireshk@...nel.org, quic_vbadigan@...cinc.com, quic_skananth@...cinc.com,
 quic_nitegupt@...cinc.com, linux-pci@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 5/6] arm64: dts: qcom: sm8450: Add opp table support to
 PCIe

On 31.01.2024 06:23, Viresh Kumar wrote:
> On 30-01-24, 18:46, Manivannan Sadhasivam wrote:
>> Agree. But what I'm saying is, right now there is no DT property in the
>> interconnect consumer nodes to specificy the bw requirements. This is all
>> hardcoded in the respective ICC consumer drivers.
> 
> I thought there are a lot of users already in there..
> 
> $ git grep -i opp.*bps arch/arm64/boot/dts/ | wc -l
> 864
> 
>> But when we use OPP to control bw, the bw requirements come from DT. This is
>> what I see as a difference. Because, only nodes making use of OPP will specify
>> bw in DT and other nodes making use of just ICC will not.
>>
>> Maybe I'm worrying too much about these details... But it looks like
>> inconsistency to me.
> 
> Right. So is there inconsistency right now ? Yes, there is.
> 
> The important question we need to answer is where do we want to see
> all these drivers (specially new ones) in the future. What's the right
> thing to do eventually ? Hardcode stuff ? Or Move it to DT ?
> 
> The answer is DT for me, so the code can be generic enough to be
> reused. This is just one step in the right direction I guess.
> Eventually the drivers must get simplified, which they are I guess.

I'm lukewarm on this.

A *lot* of hardware has more complex requirements than "x MBps at y MHz",
especially when performance counters come into the picture for dynamic
bw management.

OPP tables can't really handle this properly.

Konrad