linux-kernel - Re: [PATCH 00/10] Add PCIe Bandwidth Controller

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Date:   Mon, 11 Sep 2023 09:14:07 -0700
From:   srinivas pandruvada <srinivas.pandruvada@...ux.intel.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Krishna Chaitanya Chundru <quic_krichai@...cinc.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Zhang Rui <rui.zhang@...el.com>, linux-pm@...r.kernel.org
Cc:     linux-pci@...r.kernel.org, Bjorn Helgaas <helgaas@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Lukas Wunner <lukas@...ner.de>,
        Alexandru Gagniuc <mr.nuke.me@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alex Deucher <alexdeucher@...il.com>
Subject: Re: [PATCH 00/10] Add PCIe Bandwidth Controller

On Mon, 2023-09-11 at 18:47 +0300, Ilpo Järvinen wrote:
> + thermal people.
> 
> 

...

> Hi,
> 
> Okay, thanks for the clarification. So the point is to plan for
> adding 
> support for Link Width later and currently only support throttling
> Link 
> Speed. In any case, the Link Width control seems to be controlled
> using 
> a different approach (Link Width change does not require Link
> Retraining).
> 
> I don't know either how such 2 dimensioned throttling (Link Speed and
> Link Width) is supposed to be realized using the thermal/cooling
> device 
> interface which only provides a single integer as the current state.
> That 
> is, whether to provide a single cooling device (with a single integer
> exposed to userspace) or separate cooling device for each dimension?
> 
> Perhaps thermal people could provide some insight on this? Is there
> some 
> precedent I could take look at?
Yes. The processor cooling device does similar. 1-3 are reserved for P-
state and and 4-7 for T-states.

But I don't suggest using such method. This causes confusion and
difficult to change. For example if we increase range of P-state
control, then there is no way to know what is the start point of T-
states.

It is best to create to separate cooling devices for BW and link width.

Also there is a requirement that anything you add to thermal sysfs, it
should have some purpose for thermal control. I hope Link width control
is targeted to similar use case BW control.

Thanks,
Srinivas


>