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Date: Thu, 15 Oct 2026 15:01:52 -0500
From: Bjorn Helgaas <helgaas@kernel.org>
To: =?utf-8?B?TWljaGHFgiBDxYJhcGnFhHNraQ==?= <mclapinski@google.com>
Cc: Lukas Wunner <lukas@wunner.de>, Bjorn Helgaas <bhelgaas@google.com>,
	linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
	Jonathan Cameron <jic23@kernel.org>,
	Alistair Francis <alistair@alistair23.me>
Subject: Re: [PATCH] PCI/DOE: Poll briefly for a response before sleeping
Message-ID: <20261015200152.GA1793040@bhelgaas>
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[+cc Jonathan, Alistair, thread at
https://lore.kernel.org/all/20260917230323.3701743-1-mclapinski@google.com]

On Fri, Sep 18, 2026 at 12:28:21PM -0700, Michał Cłapiński wrote:
> On Fri, Sep 18, 2026 at 7:27 AM Lukas Wunner <lukas@wunner.de> wrote:
> >
> > On Fri, Sep 18, 2026 at 01:03:23AM +0200, Michal Clapinski wrote:
> > > doe_statemachine_work() waits for a response by sleeping
> > > PCI_DOE_POLL_INTERVAL (PCI_DOE_TIMEOUT / 128) between status reads,
> > > which is 7 ms at HZ=1000 and 4 ms at HZ=250. Exchanges are served from
> > > the device's local mailbox logic and typically complete within tens of
> > > microseconds, so nearly all of that sleep is wasted. Every DOE-capable
> > > endpoint pays it once per supported feature during enumeration.
> > >
> > > Poll with usleep_range(20, 50) for the first ~2 ms before falling back
> > > to the jiffies-based wait. On an Intel Emerald Rapids system, time spent
> > > in pci_device_add() for a DOE-capable endpoint drops from 41.8 ms to
> > > 14.0 ms.
> >
> > Polling is just a poor man's replacement for interrupts.  DOE supports
> > interrupt-driven transfers but we're not taking advantage of it so far.
> >
> > Perhaps you could look into adding that?  It would seem more worthwhile
> > than optimizing polling.
> 
> Thanks for the quick response.
> 
> This code runs during PCI device enumeration, before a driver is
> bound, so the core doesn't really have an interrupt to work with:
> MSI/MSI-X vectors belong to the driver, and enabling them in the
> core would collide with its later pci_alloc_irq_vectors(); INTx
> isn't routed yet either, since dev->irq only becomes valid via
> acpi_pci_irq_enable() at pci_enable_device() time.

Do we have to run the DOE setup code during enumeration at all?  Maybe
we can wait until a user that needs DOE functionality comes along?