Re: [RFT][PATCH] PCI: PM: Add special case handling for PCIe device wakeup

[Mika Westerberg]

Hi Rafael,

On Mon, Jul 12, 2021 at 07:06:47PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> 
> Some PCIe devices only support PME (Power Management Event) from
> D3cold.  One example is the ASMedia xHCI controller:
> 
>  11:00.0 USB controller: ASMedia Technology Inc. ASM1042A USB 3.0 Host Controller (prog-if 30 [XHCI])
>    ...
>    Capabilities: [78] Power Management version 3
>            Flags: PMEClk- DSI- D1- D2- AuxCurrent=55mA PME(D0-,D1-,D2-,D3hot-,D3cold+)
>            Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=0 PME-
> 
> In those cases, if the device is expected to generate wakeup events
> from its final power state, pci_target_state() returns D0, which
> prevents the PCIe port the device is connected to from entering any
> low-power states too.  However, if the device were allowed to go into
> D3hot, its parent PCIe port would also be able to go into D3 and if
> it goes into D3cold, it would cause the endpoint device to end up in
> D3cold too (as per the PCI PM spec v1.2, Table 6-1), in which case
> the endpoint would be able to signal PME.  This means that the system
> could be put into a lower-power configuration without sacrificing the
> the given device's ability to generate PME.
> 
> In order to avoid missing that opportunity, extend pci_pme_capable()
> to check the device's parent in the special case when the target
> state is D3hot and the device can only signal PME from D3cold and
> update pci_target_state() to return the current target state if
> pci_pme_capable() returns 'true' for it.

Thanks a lot for this!

I tried the patch and unfortunately it does not solve the issue but I
think I know what the problem is, see below.

> Link: https://lore.kernel.org/linux-pm/20210617123653.58640-1-mika.westerberg@xxxxxxxxxxxxxxx
> Reported-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
> Reported-by: Utkarsh H Patel <utkarsh.h.patel@xxxxxxxxx>
> Reported-by: Koba Ko <koba.ko@xxxxxxxxxxxxx>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> ---
> 
> Hi,
> 
> Anyone who can reproduce the problem described in the changelog,
> please test the patch and let me know the result.
> 
> Thanks!
> 
> ---
>  drivers/pci/pci.c |   38 ++++++++++++++++++++++++++------------
>  1 file changed, 26 insertions(+), 12 deletions(-)
> 
> Index: linux-pm/drivers/pci/pci.c
> ===================================================================
> --- linux-pm.orig/drivers/pci/pci.c
> +++ linux-pm/drivers/pci/pci.c
> @@ -2298,10 +2298,29 @@ void pci_pme_wakeup_bus(struct pci_bus *
>   */
>  bool pci_pme_capable(struct pci_dev *dev, pci_power_t state)
>  {
> +	struct pci_dev *parent;
> +
>  	if (!dev->pm_cap)
>  		return false;
>  
> -	return !!(dev->pme_support & (1 << state));
> +	if (dev->pme_support & (1 << state))
> +		return true;
> +
> +	/*
> +	 * Special case: The target state is D3hot and the device only supports
> +	 * signaling PME from D3cold, but it is a PCIe device whose parent port
> +	 * can go into D3cold.  In that case, if the device is allowed to go
> +	 * into D3hot, the parent port can go into D3cold which will cause the
> +	 * device to end up in D3cold, so it will be able to signal PME from the
> +	 * final state.
> +	 */
> +	if (state != PCI_D3hot || !(dev->pme_support & (1 << PCI_D3cold)))
> +		return false;
> +
> +	parent = dev->bus->self;
> +	return pci_bridge_d3_possible(parent) &&
> +		platform_pci_power_manageable(parent) &&

Here. We cannot assume that the parent has ACPI node either. For
instance when PCIe is tunneled over TBT/USB4 there can be several PCIe
switches between the problem endpoint and the root port (which has the
ACPI node of course). I think the safe assumption is to check for the
root port and whether platform_pci_power_manageable() returns true for
it but I may be missing something.

> +		platform_pci_choose_state(parent) == PCI_D3cold;
>  }
>  EXPORT_SYMBOL(pci_pme_capable);
>  
> @@ -2595,17 +2614,12 @@ static pci_power_t pci_target_state(stru
>  	if (dev->current_state == PCI_D3cold)
>  		target_state = PCI_D3cold;
>  
> -	if (wakeup) {
> -		/*
> -		 * Find the deepest state from which the device can generate
> -		 * PME#.
> -		 */
> -		if (dev->pme_support) {
> -			while (target_state
> -			      && !(dev->pme_support & (1 << target_state)))
> -				target_state--;
> -		}
> -	}
> +	if (!wakeup || !dev->pme_support || pci_pme_capable(dev, target_state))
> +		return target_state;
> +
> +	/* Find the deepest state from which the device can generate PME#. */
> +	while (target_state && !(dev->pme_support & (1 << target_state)))
> +		target_state--;
>  
>  	return target_state;
>  }
> 
>