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

[Bjorn Helgaas]

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.

> +	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.

> +
> +	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.

Bjorn