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

[Michał Cłapiński]

On Fri, Oct 16, 2026 at 10:17 PM David Matlack <dmatlack@xxxxxxxxxx> wrote:
>
> On 2026-10-12 01:42 PM, 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.
>
> Does any of this remaining time come from pci_doe_abort()? I see that is
> also called during mailbox creation and has the same call to
> pci_doe_wait(). If so, maybe the poll logic can be shared.

Yes, but I'm not sure if the poll logic can be shared without making
the code worse rather than better. I'll look into that.

Also, I'm planning on skipping pci_doe_abort for preserved devices so
it was less of a priority for me.

> >
> > Signed-off-by: Michal Clapinski <mclapinski@xxxxxxxxxx>
> > ---
> > I was asked if I can do this with interrupts instead. I don't see a way
> > to do that during device enumeration that won't collide with the
> > drivers.
> > ---
> >  drivers/pci/doe.c | 28 +++++++++++++++++++++++-----
> >  1 file changed, 23 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/pci/doe.c b/drivers/pci/doe.c
> > index 6b5d9ba8e969..5e93d78a1a97 100644
> > --- a/drivers/pci/doe.c
> > +++ b/drivers/pci/doe.c
> > @@ -27,6 +27,9 @@
> >  /* Timeout of 1 second from 6.30.2 Operation, PCI Spec r6.0 */
> >  #define PCI_DOE_TIMEOUT HZ
> >  #define PCI_DOE_POLL_INTERVAL        (PCI_DOE_TIMEOUT / 128)
>
> It would be nice to also fix the HZ=100 issue, in a separate patch
> within the same series, since you are updating this code.

Okay, will do in v2.

> > +#define PCI_DOE_POLL_FAST_MIN_US     20
> > +#define PCI_DOE_POLL_FAST_MAX_US     50
> > +#define PCI_DOE_POLL_FAST_TRIES              40
> >
> >  #define PCI_DOE_FLAG_CANCEL  0
> >  #define PCI_DOE_FLAG_DEAD    1
> > @@ -487,7 +490,7 @@ static void doe_statemachine_work(struct work_struct *work)
> >       int offset = doe_mb->cap_offset;
> >       unsigned long timeout_jiffies;
> >       u32 val;
> > -     int rc;
> > +     int rc, tries = 0;
> >
> >       if (test_bit(PCI_DOE_FLAG_DEAD, &doe_mb->flags)) {
> >               signal_task_complete(task, -EIO);
> > @@ -525,10 +528,25 @@ static void doe_statemachine_work(struct work_struct *work)
> >                       signal_task_abort(task, -EIO);
> >                       return;
> >               }
> > -             rc = pci_doe_wait(doe_mb, PCI_DOE_POLL_INTERVAL);
> > -             if (rc) {
> > -                     signal_task_abort(task, rc);
> > -                     return;
> > +             /*
> > +              * Exchanges are served from the device's local mailbox
> > +              * logic and usually complete in tens of microseconds, far
> > +              * below the jiffies granularity of PCI_DOE_POLL_INTERVAL.
> > +              * Poll for up to ~2 ms before falling back to it.
> > +              */
>
> If tens of microsecond is the norm then is polling for up to 2ms
> overkill?

I guess. I can do more precise measurements and tweak this value;
however, I can only speak for the device I have on hand, so generally
I prefer overkilling it.

> > +             if (tries++ < PCI_DOE_POLL_FAST_TRIES) {
> > +                     if (test_bit(PCI_DOE_FLAG_CANCEL, &doe_mb->flags)) {
> > +                             signal_task_abort(task, -EIO);
> > +                             return;
> > +                     }
> > +                     usleep_range(PCI_DOE_POLL_FAST_MIN_US,
> > +                                  PCI_DOE_POLL_FAST_MAX_US);
> > +             } else {
> > +                     rc = pci_doe_wait(doe_mb, PCI_DOE_POLL_INTERVAL);
> > +                     if (rc) {
> > +                             signal_task_abort(task, rc);
> > +                             return;
> > +                     }
> >               }
> >               goto retry_resp;
> >       }
> > --
> > 2.56.0.385.gd3acb90ef8-goog
> >