[v2,05/13] PCI/ASPM: Add pci_enable_link_state()

pci_disable_link_state() lacks a symmetric pair. Some drivers want to
disable ASPM during certain phases of their operation but then
re-enable it later on. If pci_disable_link_state() is made for the
device, there is currently no way to re-enable the states that were
disabled.

Add pci_enable_link_state() to remove ASPM states from the state
disable mask.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
---
 drivers/pci/pcie/aspm.c | 42 +++++++++++++++++++++++++++++++++++++++++
 include/linux/pci.h     |  2 ++
 2 files changed, 44 insertions(+)

On Mon, Sep 18, 2023 at 04:10:55PM +0300, Ilpo Järvinen wrote:
> pci_disable_link_state() lacks a symmetric pair. Some drivers want to
> disable ASPM during certain phases of their operation but then
> re-enable it later on. If pci_disable_link_state() is made for the
> device, there is currently no way to re-enable the states that were
> disabled.

pci_disable_link_state() gives drivers a way to disable specified ASPM
states using a bitmask (PCIE_LINK_STATE_L0S, PCIE_LINK_STATE_L1,
PCIE_LINK_STATE_L1_1, etc), but IIUC the driver can't tell exactly
what changed and can't directly restore the original state, e.g.,

  - PCIE_LINK_STATE_L1 enabled initially
  - driver calls pci_disable_link_state(PCIE_LINK_STATE_L0S)
  - driver calls pci_enable_link_state(PCIE_LINK_STATE_L0S)
  - PCIE_LINK_STATE_L0S and PCIE_LINK_STATE_L1 are enabled now

Now PCIE_LINK_STATE_L0S is enabled even though it was not initially
enabled.  Maybe that's what we want; I dunno.

pci_disable_link_state() currently returns success/failure, but only
r8169 and mt76 even check, and only rtl_init_one() (r8169) has a
non-trivial reason, so it's conceivable that it could return a bitmask
instead.

> Add pci_enable_link_state() to remove ASPM states from the state
> disable mask.
> 
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
> ---
>  drivers/pci/pcie/aspm.c | 42 +++++++++++++++++++++++++++++++++++++++++
>  include/linux/pci.h     |  2 ++
>  2 files changed, 44 insertions(+)
> 
> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> index 91dc95aca90f..f45d18d47c20 100644
> --- a/drivers/pci/pcie/aspm.c
> +++ b/drivers/pci/pcie/aspm.c
> @@ -1117,6 +1117,48 @@ int pci_disable_link_state(struct pci_dev *pdev, int state)
>  }
>  EXPORT_SYMBOL(pci_disable_link_state);
>  
> +/**
> + * pci_enable_link_state - Re-enable device's link state
> + * @pdev: PCI device
> + * @state: ASPM link states to re-enable
> + *
> + * Enable device's link state that were previously disable so the link is

"state[s] that were previously disable[d]" alludes to the use case you
have in mind, but I don't think it describes how this function
actually works.  This function just makes it possible to enable the
specified states.  The @state parameter may have nothing to do with
any previously disabled states.

> + * allowed to enter the specific states. Note that if the BIOS didn't grant
> + * ASPM control to the OS, this does nothing because we can't touch the
> + * LNKCTL register.
> + *
> + * Return: 0 or a negative errno.
> + */
> +int pci_enable_link_state(struct pci_dev *pdev, int state)
> +{
> +	struct pcie_link_state *link = pcie_aspm_get_link(pdev);
> +
> +	if (!link)
> +		return -EINVAL;
> +	/*
> +	 * A driver requested that ASPM be enabled on this device, but
> +	 * if we don't have permission to manage ASPM (e.g., on ACPI
> +	 * systems we have to observe the FADT ACPI_FADT_NO_ASPM bit and
> +	 * the _OSC method), we can't honor that request.
> +	 */
> +	if (aspm_disabled) {
> +		pci_warn(pdev, "can't enable ASPM; OS doesn't have ASPM control\n");
> +		return -EPERM;
> +	}
> +
> +	mutex_lock(&aspm_lock);
> +	link->aspm_disable &= ~pci_link_state_mask(state);
> +	pcie_config_aspm_link(link, policy_to_aspm_state(link));
> +
> +	if (state & PCIE_LINK_STATE_CLKPM)
> +		link->clkpm_disable = 0;
> +	pcie_set_clkpm(link, policy_to_clkpm_state(link));
> +	mutex_unlock(&aspm_lock);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(pci_enable_link_state);
> +
>  /**
>   * pci_set_default_link_state - Set the default device link state
>   * @pdev: PCI device
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 3c24ca164104..844d09230264 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -1776,11 +1776,13 @@ extern bool pcie_ports_native;
>  int pci_disable_link_state(struct pci_dev *pdev, int state);
>  int pci_disable_link_state_locked(struct pci_dev *pdev, int state);
>  #ifdef CONFIG_PCIEASPM
> +int pci_enable_link_state(struct pci_dev *pdev, int state);
>  int pci_set_default_link_state(struct pci_dev *pdev, int state);
>  void pcie_no_aspm(void);
>  bool pcie_aspm_support_enabled(void);
>  bool pcie_aspm_enabled(struct pci_dev *pdev);
>  #else
> +static inline int pci_enable_link_state(struct pci_dev *pdev, int state) { return -EOPNOTSUPP; }
>  static inline int pci_set_default_link_state(struct pci_dev *pdev, int state)
>  { return 0; }
>  static inline void pcie_no_aspm(void) { }
> -- 
> 2.30.2
> 
> 
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