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.
> > ...
>
> > +#ifndef CONFIG_PCIEASPM
> > +/*
> > + * Always disable ASPM when requested, even when CONFIG_PCIEASPM is
> > + * not build to avoid drivers adding code to do it on their own
> > + * which caused issues when core does not know about the out-of-band
> > + * ASPM state changes.
> > + */
> > +int pci_disable_link_state_locked(struct pci_dev *pdev, int state)
> > +{
> > + struct pci_dev *parent = pdev->bus->self;
> > + struct pci_bus *linkbus = pdev->bus;
> > + struct pci_dev *child;
> > + u16 aspm_enabled, linkctl;
> > + int ret;
> > +
> > + if (!parent)
> > + return -ENODEV;
>
> P.S. I think this should look the same to the user (same dmesg log and
> same taint, if we do that) as the CONFIG_PCIEASPM=y case.
Okay.
> > + ret = pcie_capability_read_word(parent, PCI_EXP_LNKCTL, &linkctl);
> > + if (ret != PCIBIOS_SUCCESSFUL)
> > + return pcibios_err_to_errno(ret);
> > + aspm_enabled = linkctl & PCI_EXP_LNKCTL_ASPMC;
> > +
> > + ret = pcie_capability_read_word(pdev, PCI_EXP_LNKCTL, &linkctl);
> > + if (ret != PCIBIOS_SUCCESSFUL)
> > + return pcibios_err_to_errno(ret);
> > + aspm_enabled |= linkctl & PCI_EXP_LNKCTL_ASPMC;
> > +
> > + /* If no states need to be disabled, don't touch LNKCTL */
> > + if (state & aspm_enabled)
> > + return 0;
> > +
> > + ret = pcie_capability_clear_word(parent, PCI_EXP_LNKCTL, PCI_EXP_LNKCTL_ASPMC);
> > + if (ret != PCIBIOS_SUCCESSFUL)
> > + return pcibios_err_to_errno(ret);
> > + list_for_each_entry(child, &linkbus->devices, bus_list)
> > + pcie_capability_clear_word(child, PCI_EXP_LNKCTL, PCI_EXP_LNKCTL_ASPMC);
>
> 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).
> > +
> > + return 0;
> > +}
>
> 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.
--
i.
