On Fri, Aug 18, 2023 at 9:40 PM Mario Limonciello <mario.limonciello@xxxxxxx> wrote: > > In Windows, systems that support Modern Standby specify hardware > pre-conditions for the SoC to be able to achieve the lowest power state > by using 'device constraints' in a SOC specific "Power Engine > Plugin" (PEP) [1] [2]. > > For each device, the constraint is the minimum (shallowest) power > state in which the device can be for the platform to be still able to > achieve significant energy conservation in a system-wide low-power > idle configuration. > > Device constraints are specified in the return value for a _DSM of > a PNP0D80 device, and Linux enumerates the constraints during probing. > > For PCI bridges (including PCIe ports), the constraints may be > regarded as an additional source of information regarding the power > state to put the device into when it is suspended. In particular, if > the constraint for a given PCI bridge is D3hot, the platform regards > D3hot as a valid power state for the bridge and it is reasonable to > expect that there won't be any side effects caused by putting the > bridge into that power state. > > Accordingly, take the low-power S0 idle (LPS0) constraints into > account when deciding whether or not to allow a given PCI bridge to be > put into D3. > > Link: https://learn.microsoft.com/en-us/windows-hardware/design/device-experiences/platform-design-for-modern-standby#low-power-core-silicon-cpu-soc-dram [1] > Link: https://uefi.org/sites/default/files/resources/Intel_ACPI_Low_Power_S0_Idle.pdf [2] > Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx> All feedback addressed AFAICS, so Reviewed-by: Rafael J. Wysocki <rafael@xxxxxxxxxx> > --- > v13->v14: > * Move code as another way to satisfy acpi_pci_bridge_d3() instead > * Update commit with Rafael's suggestions > v12->v13: > * Move back to PCI code > * Trim commit message > v11->v12: > * Adjust for dropped patch 8/9 from v11. > * Update comment > v10->v11: > * Fix kernel kernel build robot errors and various sparse warnings > related to new handling of pci_power_t. > * Use the new helpers introduced in previous patches > --- > drivers/pci/pci-acpi.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c > index 64e6ada024235..8331aea22d327 100644 > --- a/drivers/pci/pci-acpi.c > +++ b/drivers/pci/pci-acpi.c > @@ -1022,6 +1022,15 @@ bool acpi_pci_bridge_d3(struct pci_dev *dev) > > adev = ACPI_COMPANION(&dev->dev); > if (adev) { > + int acpi_state; > + > + /* Check if the platform specifies an LPS0 constraint of D3. */ > + acpi_state = acpi_get_lps0_constraint(adev); > + pci_dbg(dev, "LPS0 constraint: %d\n", acpi_state); > + if (acpi_state != ACPI_STATE_UNKNOWN && > + acpi_state >= ACPI_STATE_D3_HOT) > + return true; > + > /* > * If the bridge has _S0W, whether or not it can go into D3 > * depends on what is returned by that object. In particular, > -- > 2.34.1 >
