Re: [PATCH] PCI/DOE: Poll briefly for a response before sleeping

[Michał Cłapiński]

On Fri, Sep 18, 2026 at 7:27 AM Lukas Wunner <lukas@xxxxxxxxx> 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.