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

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Message-ID: <20250616210530.GA1106466@bhelgaas>
Date: Mon, 16 Jun 2025 16:05:30 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Graham Whyte <grwhyte@...ux.microsoft.com>
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 Mon, Jun 16, 2025 at 12:02:41PM -0700, Graham Whyte wrote:
> 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.
> 
> 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).

Sure.  That's a trivial change.  The problem is the quirk itself.

The Readiness Time Reporting Extended Capability is read-only with no
control bits in it so it requires no actual logic in the device.
Maybe you can just implement that capability with a firmware change on
the device and add the corresponding Linux support for it.