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

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Date:   Tue, 22 Oct 2019 07:59:50 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Dilip Kota <eswara.kota@...ux.intel.com>
Cc:     Andrew Murray <andrew.murray@....com>, jingoohan1@...il.com,
        gustavo.pimentel@...opsys.com, lorenzo.pieralisi@....com,
        robh@...nel.org, martin.blumenstingl@...glemail.com,
        linux-pci@...r.kernel.org, hch@...radead.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        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@...r.kernel.org
Subject: Re: [PATCH v4 3/3] pci: intel: Add sysfs attributes to configure
 pcie link

[+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.

Bjorn