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

From: Bjorn Helgaas <[email-masked]>
Date: Wed, 11 Oct 2023 15:04:42 -0500

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.
> 
> A number of drivers have added workarounds to force ASPM off with own
> writes into the Link Control Register (some even with comments
> explaining why PCI core does not disable it under some circumstances).
> According to the comments, some drivers require ASPM to be off for
> reliable operation.
> 
> Having custom ASPM handling in drivers is problematic because the state
> kept in the ASPM service driver is not updated by the changes made
> outside the link state management API.
> 
> As the first step to address this issue, make pci_disable_link_state()
> to unconditionally disable ASPM so the motivation for drivers to come
> up with custom ASPM handling code is eliminated.
> 
> Place the minimal ASPM disable handling into own file as it is too
> complicated to fit into a header as static inline and it has almost no
> overlap with the existing, more complicated ASPM code in
> drivers/pci/pce/aspm.c.
> 
> Make pci_disable_link_state() function comment to comply kerneldoc
> formatting while changing the description.
> 
> Link: https://lore.kernel.org/all/[email-masked]/
> Link: https://lore.kernel.org/all/[email-masked]/
> Signed-off-by: Ilpo Järvinen <[email-masked]>
> ---
>  drivers/pci/pcie/Makefile       |  1 +
>  drivers/pci/pcie/aspm.c         | 33 ++++++++++-------
>  drivers/pci/pcie/aspm_minimal.c | 66 +++++++++++++++++++++++++++++++++
>  include/linux/pci.h             |  6 +--
>  4 files changed, 88 insertions(+), 18 deletions(-)
>  create mode 100644 drivers/pci/pcie/aspm_minimal.c
> 
> diff --git a/drivers/pci/pcie/Makefile b/drivers/pci/pcie/Makefile
> index 8de4ed5f98f1..ec7f04037b01 100644
> --- a/drivers/pci/pcie/Makefile
> +++ b/drivers/pci/pcie/Makefile
> @@ -6,6 +6,7 @@ pcieportdrv-y			:= portdrv.o rcec.o
>  
>  obj-$(CONFIG_PCIEPORTBUS)	+= pcieportdrv.o
>  
> +obj-y				+= aspm_minimal.o

Can we put this code in drivers/pci/pci.c instead of creating a new
file for it?  pci.c is kind of a dumping ground and isn't ideal
either, but we do have a few other things there that we *always* want
even though they're related to a separate Kconfig feature, e.g.,
pci_bridge_reconfigure_ltr(), pcie_clear_device_status(),
pcie_clear_root_pme_status().

>  obj-$(CONFIG_PCIEASPM)		+= aspm.o

Or maybe it would be better to just put it in aspm.c, drop this
compilation guard, and wrap the rest of the file in #ifdef
CONFIG_PCIEASPM.  Then everything would be in one file, which is a
major boon for code readers.

What do you think?

>  obj-$(CONFIG_PCIEAER)		+= aer.o err.o
>  obj-$(CONFIG_PCIEAER_INJECT)	+= aer_inject.o
> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> index 860bc94974ec..ec6d7a092ac1 100644
> --- a/drivers/pci/pcie/aspm.c
> +++ b/drivers/pci/pcie/aspm.c
> @@ -1042,16 +1042,23 @@ static int __pci_disable_link_state(struct pci_dev *pdev, int state, bool sem)
>  		return -EINVAL;
>  	/*
>  	 * A driver requested that ASPM be disabled on this device, but
> -	 * if we don't have permission to manage ASPM (e.g., on ACPI
> +	 * if we might not 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.  Windows has
> -	 * a similar mechanism using "PciASPMOptOut", which is also
> -	 * ignored in this situation.
> +	 * the _OSC method), previously we chose to not honor disable
> +	 * request in that case. Windows has a similar mechanism using
> +	 * "PciASPMOptOut", which is also ignored in this situation.
> +	 *
> +	 * Not honoring the requests to disable ASPM, however, led to
> +	 * drivers forcing ASPM off on their own. As such changes of ASPM
> +	 * state are not tracked by this service driver, the state kept here
> +	 * became out of sync.
> +	 *
> +	 * Therefore, honor ASPM disable requests even when OS does not have
> +	 * ASPM control. Plain disable for ASPM is assumed to be slightly
> +	 * safer than fully managing it.
>  	 */
> -	if (aspm_disabled) {
> -		pci_warn(pdev, "can't disable ASPM; OS doesn't have ASPM control\n");
> -		return -EPERM;
> -	}
> +	if (aspm_disabled)
> +		pci_warn(pdev, "OS doesn't have ASPM control, disabling ASPM anyway\n");

I think this is better than the previous situation, but I think we
should taint the kernel here because it's possible the firmware had a
reason for retaining ASPM control, so we might be stepping on
something.  Arguably the message is already enough of a signal, but
checking for a taint is potentially a little more automatable.

> +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;
> +
> +	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;

In this case, we don't care about the shift offset of the
PCI_EXP_LNKCTL_ASPMC bitfield, but if we use FIELD_GET() in most/all
other cases where we look at PCI_EXP_LNKCTL, maybe it would be worth
using it here as well?

Tangent, but I'm always dubious about the idea that e1000e is so
special that only there do we need the "_locked" variant of this
function.  No suggestion though; no need to do anything about it in
this series ;)

> +	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);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(pci_disable_link_state_locked);