linux-kernel - Re: [PATCH v4 3/3] pci: intel: Add sysfs attributes to configure pcie link

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Date:   Tue, 29 Oct 2019 11:42:53 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Dilip Kota <eswara.kota@...ux.intel.com>,
        Andrew Murray <andrew.murray@....com>,
        Jingoo Han <jingoohan1@...il.com>,
        gustavo.pimentel@...opsys.com,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        martin.blumenstingl@...glemail.com,
        Linux PCI <linux-pci@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Shevchenko, Andriy" <andriy.shevchenko@...el.com>,
        cheol.yong.kim@...el.com, chuanhua.lei@...ux.intel.com,
        qi-ming.wu@...el.com, "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v4 3/3] pci: intel: Add sysfs attributes to configure pcie link

On Tue, Oct 22, 2019 at 2:59 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
>
> [+cc Rafael, linux-pm, beginning of discussion at
> https://lore.kernel.org/r/d8574605f8e70f41ce1e88ccfb56b63c8f85e4df.1571638827.git.eswara.kota@linux.intel.com]
>
> On Tue, Oct 22, 2019 at 05:27:38PM +0800, Dilip Kota wrote:
> > On 10/22/2019 1:18 AM, Bjorn Helgaas wrote:
> > > On Mon, Oct 21, 2019 at 02:38:50PM +0100, Andrew Murray wrote:
> > > > On Mon, Oct 21, 2019 at 02:39:20PM +0800, Dilip Kota wrote:
> > > > > PCIe RC driver on Intel Gateway SoCs have a requirement
> > > > > of changing link width and speed on the fly.
> > > Please add more details about why this is needed.  Since you're adding
> > > sysfs files, it sounds like it's not actually the *driver* that needs
> > > this; it's something in userspace?
>
> > We have use cases to change the link speed and width on the fly.
> > One is EMI check and other is power saving.  Some battery backed
> > applications have to switch PCIe link from higher GEN to GEN1 and
> > width to x1. During the cases like external power supply got
> > disconnected or broken. Once external power supply is connected then
> > switch PCIe link to higher GEN and width.
>
> That sounds plausible, but of course nothing there is specific to the
> Intel Gateway, so we should implement this generically so it would
> work on all hardware.
>
> I'm not sure what the interface should look like -- should it be a
> low-level interface as you propose where userspace would have to
> identify each link of interest, or is there some system-wide
> power/performance knob that could tune all links?  Cc'd Rafael and
> linux-pm in case they have ideas.

Frankly, I need some time to think about this and, in case you are
wondering about whether or not it has been discussed with me already,
it hasn't.

At this point I can only say that since we have an ASPM interface,
which IMO is not fantastic, it may be good to come up with a common
link management interface.

Cheers!