Re: [PATCH] Use maximum latency when determining L1 ASPM

[Ian Kumlien <ian.kumlien@gmail.com>]

On Thu, Jul 30, 2020 at 12:28 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
> On Mon, Jul 27, 2020 at 11:30:45PM +0200, Ian Kumlien wrote:
> > Currently we check the maximum latency of upstream and downstream
> > per link, not the maximum for the path
> >
> > This would work if all links have the same latency, but:
> > endpoint -> c -> b -> a -> root  (in the order we walk the path)
> >
> > If c or b has the higest latency, it will not register
> >
> > Fix this by maintaining the maximum latency value for the path
> >
> > This change fixes a regression introduced by:
> > 66ff14e59e8a (PCI/ASPM: Allow ASPM on links to PCIe-to-PCI/PCI-X Bridges)
>
> Hi Ian,
>
> Sorry about the regression, and thank you very much for doing the
> hard work of debugging and fixing it!
>
> My guess is that 66ff14e59e8a isn't itself buggy, but it allowed ASPM
> to be enabled on a longer path, and we weren't computing the maximum
> latency correctly, so ASPM on that longer path exceeded the amount we
> could tolerate.  If that's the case, 66ff14e59e8a probably just
> exposed an existing problem that could occur in other topologies even
> without 66ff14e59e8a.

I agree, this is why I didn't do fixes:. but it does fix a regression
- and it's hard to say
exactly what - I'd like it to go in to stable when accepted...

But we can rewrite it anyway you see fit :)

> I'd like to work through this latency code with concrete examples.
> Can you collect the "sudo lspci -vv" output and attach it to an entry
> at https://bugzilla.kernel.org?  If it's convenient, it would be
> really nice to compare it with similar output from before this patch.

I can cut from the mails i had in the conversation with Alexander Duyck

I submitted it against PCI, you can find it here:
https://bugzilla.kernel.org/show_bug.cgi?id=208741

Still filling in the data

> Bjorn
>
> > Signed-off-by: Ian Kumlien <ian.kumlien@gmail.com>
> > ---
> >  drivers/pci/pcie/aspm.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> > index b17e5ffd31b1..bd53fba7f382 100644
> > --- a/drivers/pci/pcie/aspm.c
> > +++ b/drivers/pci/pcie/aspm.c
> > @@ -434,7 +434,7 @@ static void pcie_get_aspm_reg(struct pci_dev *pdev,
> >
> >  static void pcie_aspm_check_latency(struct pci_dev *endpoint)
> >  {
> > -     u32 latency, l1_switch_latency = 0;
> > +     u32 latency, l1_max_latency = 0, l1_switch_latency = 0;
> >       struct aspm_latency *acceptable;
> >       struct pcie_link_state *link;
> >
> > @@ -470,8 +470,9 @@ static void pcie_aspm_check_latency(struct pci_dev *endpoint)
> >                * substate latencies (and hence do not do any check).
> >                */
> >               latency = max_t(u32, link->latency_up.l1, link->latency_dw.l1);
> > +             l1_max_latency = max_t(u32, latency, l1_max_latency);
> >               if ((link->aspm_capable & ASPM_STATE_L1) &&
> > -                 (latency + l1_switch_latency > acceptable->l1))
> > +                 (l1_max_latency + l1_switch_latency > acceptable->l1))
> >                       link->aspm_capable &= ~ASPM_STATE_L1;
> >               l1_switch_latency += 1000;
> >
> > --
> > 2.27.0
> >