linux-kernel - Re: [PATCH v3 0/2] PCI: Reduce FLR delay to 10ms for MSFT devices

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Message-ID: <56b926e8-4b5c-45a8-882e-7c6be0e2fd26@linux.microsoft.com>
Date: Mon, 16 Jun 2025 12:02:41 -0700
From: Graham Whyte <grwhyte@...ux.microsoft.com>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Christoph Hellwig <hch@...radead.org>, Niklas Cassel <cassel@...nel.org>,
 linux-pci@...r.kernel.org, shyamsaini@...ux.microsoft.com, code@...icks.com,
 Okaya@...nel.org, bhelgaas@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/2] PCI: Reduce FLR delay to 10ms for MSFT devices



On 6/13/2025 8:33 AM, Bjorn Helgaas wrote:
> On Thu, Jun 12, 2025 at 09:41:45AM -0700, Graham Whyte wrote:
>> On 6/11/2025 11:31 PM, Christoph Hellwig wrote:
>>> On Wed, Jun 11, 2025 at 01:08:21PM -0700, Graham Whyte wrote:
>>>> We can ask our HW engineers to implement function readiness but we need
>>>> to be able to support exiting products, hence why posting it as a quirk.
>>>
>>> Your report sounds like it works perfectly fine, it's just that you
>>> want to reduce the delay.  For that you'll need to stick to the standard
>>> methods instead of adding quirks, which are for buggy hardware that does
>>> not otherwise work.
>>
>> Bjorn, what would you recommend as next steps here?
> 
> This is a tough call and I don't pretend to have an obvious answer.  I
> understand the desire to improve performance.  On the other hand, PCI
> has been successful over the long term because devices adhere to
> standardized ways of doing things, which makes generic software
> possible.  Quirks degrade that story, of course, especially when there
> is an existing standardized solution that isn't being used.  I'm not
> at all happy about vendors that decide against the standard solution
> and then ask OS folks to do extra work to compensate.
> 
> Bjorn

Hi Bjorn,

Should someone want to implement readiness time reporting down the road,
they'll need to do the same work as patch 1 in this series (making the
flr delay a configurable parameter). This change lays the groundwork for
that work, while also supporting devices that can't use readiness time
reporting.

Alternatively could we use a sysfs file to make this parameter
configurable via the user space application? Similar to switching between
d3hot and d3cold by writing to /sys/bus/pci/slots/$DEVICE/power.

Thanks,
Graham