linux-kernel - Re: [PATCH v2] PCI: Fix no-op wait after secondary bus reset

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Message-ID: <20220523142042.GA19286@wunner.de>
Date:   Mon, 23 May 2022 16:20:42 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Sheng Bi <windy.bi.enflame@...il.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] PCI: Fix no-op wait after secondary bus reset

On Sun, May 22, 2022 at 01:37:50AM +0800, Sheng Bi wrote:
> On Sat, May 21, 2022 at 8:49 PM Lukas Wunner <lukas@...ner.de> wrote:
> > On Sat, May 21, 2022 at 04:36:10PM +0800, Sheng Bi wrote:
> > > If so, I also want to align the polling things mentioned in the
> > > question from Alex, since pci_dev_wait() is also used for reset
> > > functions other than SBR. To Bjorn, Alex, Lucas, how do you think if
> > > we need to change the polling in pci_dev_wait() to 20ms intervals, or
> > > keep binary exponential back-off with probable unexpected extra
> > > timeout delay.
> >
> > The exponential backoff should probably be capped at some point
> > to avoid excessive wait delays.  I guess the rationale for
> > exponential backoff is to not poll too frequently.
> > Capping at 20 msec or 100 msec may be reasonable, i.e.:
> >
> > -               delay *= 2;
> > +               delay = min(delay * 2, 100);
> 
> Capping at 20 or 100 msec seems reasonable to me. Btw, since 20 msec
> is not a long time in these scenarios, how about changing to a fixed
> 20 msec interval?

The callers of pci_dev_wait() seem to wait for the spec-defined
delay and only call pci_dev_wait() to allow for an additional period
that non-compliant devices may need.  That extra delay can be expected
to be low, which is why it makes sense to start with a short poll interval
and gradually extend it.  So the algorithm seems to be reasonable and
I wouldn't recommend changing it to a constant interval unless that
fixes something which is currently broken.

Thanks,

Lukas