Re: [PATCH v2 1/6] PCI/ASPM: Add locked helper for enabling link state

Ilpo Järvinen
2023-12-12T09:26:33Z

On Fri, 8 Dec 2023, Bjorn Helgaas wrote:

> On Fri, Dec 08, 2023 at 09:00:56AM +0100, Johan Hovold wrote:
> > On Thu, Dec 07, 2023 at 02:47:16PM -0600, Bjorn Helgaas wrote:
> > > On Tue, Nov 28, 2023 at 09:15:07AM +0100, Johan Hovold wrote:
> > > > Add a helper for enabling link states that can be used in contexts where
> > > > a pci_bus_sem read lock is already held (e.g. from pci_walk_bus()).
> > > >
> > > > This helper will be used to fix a couple of potential deadlocks where
> > > > the current helper is called with the lock already held, hence the CC
> > > > stable tag.
> >
> > > As far as I can see, we end up with pci_enable_link_state() defined
> > > but never called and pci_enable_link_state_locked() being called only
> > > by pcie-qcom.c and vmd.c.
> >
> > Correct, I mentioned this in the cover letter.
>
> Ah, right. I really don't like these exported locked/unlocked
> interfaces because pci_bus_sem is internal to the PCI core, and the
> caller shouldn't need to know or be able to specify whether it is held
> or not. They exist for now, but I think we should try to get rid of
> them.
>
> > > Can we just rename pci_enable_link_state() to
> > > pci_enable_link_state_locked() and assert that pci_bus_sem is held, so
> > > we don't end up with a function that's never used?
> >
> > That would work too. I went with adding a new helper to facilitate
> > stable backports and to mirror pci_disable_link_state(). The variants
> > are simple wrappers around the implementation so there's no real cost to
> > having the unused one.
>
> Makes good sense. There's no real machine cost to the unused one; I'm
> more concerned about the human cost here.

I know these were already applied but I want to correct one small
misconcept that seems to be floating around thanks the misleading name...

pci_enable_link_state() is not really a pair/mirror of
pci_disable_link_state() despite its name. It would be better called
pci_set_default_link_state() to better match what it does.

--
i.