Re: [PATCH v2 10/10] PCI / ACPI: Whitelist D3 for more PCIe hotplug ports

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Apr 08, 2020 at 09:04:09AM +0300, Mika Westerberg wrote:
> On Tue, Apr 07, 2020 at 06:54:23PM -0500, Bjorn Helgaas wrote:
> > On Thu, Sep 13, 2018 at 05:33:22PM +0300, Mika Westerberg wrote:
> > > In order to have better power management for Thunderbolt PCIe chains,
> > > Windows enables power management for native PCIe hotplug ports if there
> > > is following ACPI _DSD attached to the root port:
> > > 
> > >   Name (_DSD, Package () {
> > >       ToUUID ("6211e2c0-58a3-4af3-90e1-927a4e0c55a4"),
> > >       Package () {
> > >           Package () {"HotPlugSupportInD3", 1}
> > >       }
> > >   })
> > > 
> > > This is also documented in:
> > > 
> > >   https://docs.microsoft.com/en-us/windows-hardware/drivers/pci/dsd-for-pcie-root-ports#identifying-pcie-root-ports-supporting-hot-plug-in-d3
> > 
> > This doc basically says that if the platform supplies this _DSD, the
> > root port is "capable of handling hot plug events while in D3 state".
> > 
> > What does that mean?  That statement is not really actionable.  I
> > *assume* it's telling us about some specific hardware or firmware
> > functionality, like maybe we'll get a notification for hotplug events
> > when the device is in D3?  D3hot?  D3cold?  What is the notification?
> > Is it immediate or when the device comes back to D0?  How do we
> > control and field the notification?
> 
> I think it simply gives the OS a hint that it can put PCIe hotplug
> capable port into D3 and expect it to wake up when device is detected.

I'd really like more specific details than this.  PCI power management
is explicitly controlled by software, so I don't know what it means
for a bridge to "wake up when device is detected."

Normally Linux would get some kind of notification like a PME, then
execute an ACPI method and/or write PCI_PM_CTRL to put the device back
in D0.

Are we talking about D3hot or D3cold?

The PCI PM capability already has a way to advertise that it can
generate PME from D3cold.  How is this different from that?

Is this _DSD something that *could* be advertised via PCI config
space, i.e., is it completely determined by the Root Port?  Or is it
something that requires ACPI support, so it cannot be done directly by
the hardware device?

These are all things we need to know in order to make power management
reliable.

> > > Do the same in Linux by introducing new firmware PM callback (->bridge_d3())
> > > and then implement it for ACPI based systems so that the above property is
> > > checked.
> > > 
> > > There is one catch, though. The initial pci_dev->bridge_d3 is set before
> > > the root port has ACPI companion bound (the device is not added to the
> > > PCI bus either) so we need to look up the ACPI companion manually in
> > > that case in acpi_pci_bridge_d3().
> > > 
> > > Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
> > > Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > > ---
> > >  drivers/acpi/property.c |  3 +++
> > >  drivers/pci/pci-acpi.c  | 41 +++++++++++++++++++++++++++++++++++++++++
> > >  drivers/pci/pci.c       |  9 +++++++++
> > >  drivers/pci/pci.h       |  3 +++
> > >  4 files changed, 56 insertions(+)
> > > 
> > > diff --git a/drivers/acpi/property.c b/drivers/acpi/property.c
> > > index 90ba9371bae6..8c7c4583b52d 100644
> > > --- a/drivers/acpi/property.c
> > > +++ b/drivers/acpi/property.c
> > > @@ -28,6 +28,9 @@ static const guid_t prp_guids[] = {
> > >  	/* ACPI _DSD device properties GUID: daffd814-6eba-4d8c-8a91-bc9bbf4aa301 */
> > >  	GUID_INIT(0xdaffd814, 0x6eba, 0x4d8c,
> > >  		  0x8a, 0x91, 0xbc, 0x9b, 0xbf, 0x4a, 0xa3, 0x01),
> > > +	/* Hotplug in D3 GUID: 6211e2c0-58a3-4af3-90e1-927a4e0c55a4 */
> > > +	GUID_INIT(0x6211e2c0, 0x58a3, 0x4af3,
> > > +		  0x90, 0xe1, 0x92, 0x7a, 0x4e, 0x0c, 0x55, 0xa4),
> > >  };
> > >  
> > >  static const guid_t ads_guid =
> > > diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
> > > index f8436d1c4d45..c8d0549580f4 100644
> > > --- a/drivers/pci/pci-acpi.c
> > > +++ b/drivers/pci/pci-acpi.c
> > > @@ -519,6 +519,46 @@ static pci_power_t acpi_pci_choose_state(struct pci_dev *pdev)
> > >  	return PCI_POWER_ERROR;
> > >  }
> > >  
> > > +static struct acpi_device *acpi_pci_find_companion(struct device *dev);
> > > +
> > > +static bool acpi_pci_bridge_d3(struct pci_dev *dev)
> > > +{
> > > +	const struct fwnode_handle *fwnode;
> > > +	struct acpi_device *adev;
> > > +	struct pci_dev *root;
> > > +	u8 val;
> > > +
> > > +	if (!dev->is_hotplug_bridge)
> > > +		return false;
> > > +
> > > +	/*
> > > +	 * Look for a special _DSD property for the root port and if it
> > > +	 * is set we know the hierarchy behind it supports D3 just fine.
> > > +	 */
> > > +	root = pci_find_pcie_root_port(dev);
> > > +	if (!root)
> > > +		return false;
> > > +
> > > +	adev = ACPI_COMPANION(&root->dev);
> > > +	if (root == dev) {
> > > +		/*
> > > +		 * It is possible that the ACPI companion is not yet bound
> > > +		 * for the root port so look it up manually here.
> > > +		 */
> > > +		if (!adev && !pci_dev_is_added(root))
> > > +			adev = acpi_pci_find_companion(&root->dev);
> > > +	}
> > > +
> > > +	if (!adev)
> > > +		return false;
> > > +
> > > +	fwnode = acpi_fwnode_handle(adev);
> > > +	if (fwnode_property_read_u8(fwnode, "HotPlugSupportInD3", &val))
> > > +		return false;
> > > +
> > > +	return val == 1;
> > > +}
> > > +
> > >  static bool acpi_pci_power_manageable(struct pci_dev *dev)
> > >  {
> > >  	struct acpi_device *adev = ACPI_COMPANION(&dev->dev);
> > > @@ -635,6 +675,7 @@ static bool acpi_pci_need_resume(struct pci_dev *dev)
> > >  }
> > >  
> > >  static const struct pci_platform_pm_ops acpi_pci_platform_pm = {
> > > +	.bridge_d3 = acpi_pci_bridge_d3,
> > >  	.is_manageable = acpi_pci_power_manageable,
> > >  	.set_state = acpi_pci_set_power_state,
> > >  	.get_state = acpi_pci_get_power_state,
> > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > index 1af6f1887986..b1b3052f15dc 100644
> > > --- a/drivers/pci/pci.c
> > > +++ b/drivers/pci/pci.c
> > > @@ -791,6 +791,11 @@ static inline bool platform_pci_need_resume(struct pci_dev *dev)
> > >  	return pci_platform_pm ? pci_platform_pm->need_resume(dev) : false;
> > >  }
> > >  
> > > +static inline bool platform_pci_bridge_d3(struct pci_dev *dev)
> > > +{
> > > +	return pci_platform_pm ? pci_platform_pm->bridge_d3(dev) : false;
> > 
> > This patch added a .bridge_d3() implementation for ACPI but not for
> > MID.  What prevents us from calling platform_pci_bridge_d3() on a MID
> > platform and trying to call through a NULL pointer?
> > 
> > Shouldn't we do something like the patch attached below?
> 
> IIRC MID devices in general don't have PCIe ports (so we never enable PM
> for them). Is this a real problem that crashes peoples kernels on MID
> systems? Then yes I think the patch makes sense to have.
> 
> I also remember testing some other stuff on one MID system (Edison)
> quite recently and did not see any issues.

I have not seen reports of crashes, but I do think this is a real
problem.  The problem is that the code as-is relies on assumptions
("MID does not have PCI and never will") that are implicit and
impossible to verify, which is a maintenance problem.

> > > +}
> > > +
> > >  /**
> > >   * pci_raw_set_power_state - Use PCI PM registers to set the power state of
> > >   *                           given PCI device
> > > @@ -2514,6 +2519,10 @@ bool pci_bridge_d3_possible(struct pci_dev *bridge)
> > >  		if (bridge->is_thunderbolt)
> > >  			return true;
> > >  
> > > +		/* Platform might know better if the bridge supports D3 */
> > > +		if (platform_pci_bridge_d3(bridge))
> > > +			return true;
> > 
> > *All* devices trivially support D3.  Obviously we're trying to learn
> > something else here.  What is it?
> 
> D3 has been problematic with hotplug bridges which is the reason we have
> not put them in D3 until recently (and still don't do that for ACPI
> hotplug bridges).
> 
> BTW, this patch was merged over a year ago so I'm not sure why you comment
> it now. Or you want me to add incremental changes to it?

I was reviewing [1], which updates pci_find_pcie_root_port(), which is
used by acpi_pci_bridge_d3().  I noticed a potential NULL pointer
dereference, which was a distraction.

Unless you object, I'll merge something like the patch below to
prevent that distraction for others.

[1] https://lore.kernel.org/r/1586262717-23566-1-git-send-email-yangyicong@hisilicon.com

> > >  		/*
> > >  		 * Hotplug ports handled natively by the OS were not validated
> > >  		 * by vendors for runtime D3 at least until 2018 because there
> > > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > > index 6e0d1528d471..66fd5c1bf71b 100644
> > > --- a/drivers/pci/pci.h
> > > +++ b/drivers/pci/pci.h
> > > @@ -39,6 +39,8 @@ int pci_bridge_secondary_bus_reset(struct pci_dev *dev);
> > >  /**
> > >   * struct pci_platform_pm_ops - Firmware PM callbacks
> > >   *
> > > + * @bridge_d3: Does the bridge allow entering into D3
> > > + *
> > >   * @is_manageable: returns 'true' if given device is power manageable by the
> > >   *		   platform firmware
> > >   *
> > > @@ -60,6 +62,7 @@ int pci_bridge_secondary_bus_reset(struct pci_dev *dev);
> > >   * these callbacks are mandatory.
> > >   */
> > >  struct pci_platform_pm_ops {
> > > +	bool (*bridge_d3)(struct pci_dev *dev);
> > >  	bool (*is_manageable)(struct pci_dev *dev);
> > >  	int (*set_state)(struct pci_dev *dev, pci_power_t state);
> > >  	pci_power_t (*get_state)(struct pci_dev *dev);
> > > -- 
> > 
> > 
> > diff --git a/drivers/pci/pci-mid.c b/drivers/pci/pci-mid.c
> > index aafd58da3a89..0bacd45b30d6 100644
> > --- a/drivers/pci/pci-mid.c
> > +++ b/drivers/pci/pci-mid.c
> > @@ -16,6 +16,11 @@
> >  
> >  #include "pci.h"
> >  
> > +static bool mid_pci_bridge_d3(struct pci_dev *dev)
> > +{
> > +	return false;
> > +}
> > +
> >  static bool mid_pci_power_manageable(struct pci_dev *dev)
> >  {
> >  	return true;
> > @@ -47,6 +52,7 @@ static bool mid_pci_need_resume(struct pci_dev *dev)
> >  }
> >  
> >  static const struct pci_platform_pm_ops mid_pci_platform_pm = {
> > +	.bridge_d3	= mid_pci_bridge_d3,
> >  	.is_manageable	= mid_pci_power_manageable,
> >  	.set_state	= mid_pci_set_power_state,
> >  	.get_state	= mid_pci_get_power_state,
> > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > index 595fcf59843f..fa837e88ea07 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -820,8 +820,9 @@ static const struct pci_platform_pm_ops *pci_platform_pm;
> >  
> >  int pci_set_platform_pm(const struct pci_platform_pm_ops *ops)
> >  {
> > -	if (!ops->is_manageable || !ops->set_state  || !ops->get_state ||
> > -	    !ops->choose_state  || !ops->set_wakeup || !ops->need_resume)
> > +	if (!ops->bridge_d3 || !ops->is_manageable || !ops->set_state  ||
> > +	    !ops->get_state || !ops->choose_state  || !ops->set_wakeup ||
> > +	    !ops->need_resume)
> >  		return -EINVAL;
> >  	pci_platform_pm = ops;
> >  	return 0;