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Subject: Re: [PATCH v4 3/3] pci: intel: Add sysfs attributes to configure pcie
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To:     Bjorn Helgaas <helgaas@kernel.org>
Cc:     Andrew Murray <andrew.murray@arm.com>, jingoohan1@gmail.com,
        gustavo.pimentel@synopsys.com, lorenzo.pieralisi@arm.com,
        robh@kernel.org, martin.blumenstingl@googlemail.com,
        linux-pci@vger.kernel.org, hch@infradead.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        andriy.shevchenko@intel.com, cheol.yong.kim@intel.com,
        chuanhua.lei@linux.intel.com, qi-ming.wu@intel.com,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>, linux-pm@vger.kernel.org
References: <20191022125950.GA133170@google.com>
From:   Dilip Kota <eswara.kota@linux.intel.com>
Message-ID: <c1aadeea-7904-1455-5393-c4998fbd8037@linux.intel.com>
Date:   Tue, 29 Oct 2019 17:31:18 +0800
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On 10/22/2019 8:59 PM, Bjorn Helgaas 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.
Agree.
>
> 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.

To my knowledge sysfs is the appropriate way to go.
If there are any other best possible knobs, will be helpful.

Regards,
Dilip

>
> Bjorn