Re: [PATCH v2 03/13] PCI/ASPM: Disable ASPM when driver requests it

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Oct 12, 2023 at 01:56:16PM +0300, Ilpo Järvinen wrote:
> On Wed, 11 Oct 2023, Bjorn Helgaas wrote:
> > On Mon, Sep 18, 2023 at 04:10:53PM +0300, Ilpo Järvinen wrote:
> > > PCI core/ASPM service driver allows controlling ASPM state through
> > > pci_disable_link_state() and pci_enable_link_state() API. It was
> > > decided earlier (see the Link below), to not allow ASPM changes when OS
> > > does not have control over it but only log a warning about the problem
> > > (commit 2add0ec14c25 ("PCI/ASPM: Warn when driver asks to disable ASPM,
> > > but we can't do it")). Similarly, if ASPM is not enabled through
> > > config, ASPM cannot be disabled.
> ...

> > This disables *all* ASPM states, unlike the version when
> > CONFIG_PCIEASPM is enabled.  I suppose there's a reason, and maybe a
> > comment could elaborate on it?
> >
> > When CONFIG_PCIEASPM is not enabled, I don't think we actively
> > *disable* ASPM in the hardware; we just leave it as-is, so firmware
> > might have left it enabled.
> 
> This whole trickery is intended for drivers that do not want to have ASPM 
> because the devices are broken with it. So leaving it as-is is not really 
> an option (as demonstrated by the custom workarounds).

Right.

> > Conceptually it seems like the LNKCTL updates here should be the same
> > whether CONFIG_PCIEASPM is enabled or not (subject to the question
> > above).
> > 
> > When CONFIG_PCIEASPM is enabled, we might need to do more stuff, but
> > it seems like the core should be the same.
> 
> So you think it's safer to partially disable ASPM (as per driver's 
> request) rather than disable it completely? I got the impression that the 
> latter might be safer from what Rafael said earlier but I suppose I might 
> have misinterpreted him since he didn't exactly say that it might be safer 
> to _completely_ disable it.

My question is whether the state of the device should depend on
CONFIG_PCIEASPM.  If the driver does this:

  pci_disable_link_state(PCIE_LINK_STATE_L0S)

do we want to leave L1 enabled when CONFIG_PCIEASPM=y but disable L1
when CONFIG_PCIEASPM is unset?

I can see arguments both ways.  My thought was that it would be nice
to end up with a single implementation of pci_disable_link_state()
with an #ifdef around the CONFIG_PCIEASPM-enabled stuff because it
makes the code easier to read.

Bjorn

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