[PATCH 03/12] PCI/ACPI: Add aux power grant notifier

[Rafael J. Wysocki]

On Fri, Apr 4, 2025 at 2:53 PM Gupta, Anshuman <anshuman.gupta at intel.com> wrote:
>
> /snip
> > > >
> > > > Exactly like I said: If you only allow one driver to use the _DSM to
> > > > request the Aux power from a given Root Port, it will have all of
> > > > the information and does not need to be notified about any changes.
> > > > Since no one else is allowed to use this interface for that Root
> > > > Port, no one else will need a notifier either.  For this to work,
> > > > you need some mechanism allowing drivers to claim the interface so
> > > > no one else can use it (on a per Root Port basis) which is currently missing
> > AFAICS.
> > >
> > > IMHO such kind of mechanism will require to add Root Port specific
> > > data structure to claim the interface. But real problem is the criteria  to claim
> > the interface.
> > > Is first PCIe Non-Bridge Endpoint Function 0 driver can be criteria
> > > to claim the Interface. Or first come and first serve approach ?
> >
> > IMV, the first PCIe Non-Bridge Endpoint Function 0 driver approach would be
> > sort of fragile and cumbersome to enforce.
> >
> > First come, first serve is much simpler and should be sufficient for now AFAICS.
>
> We are enabling VRSR only for default vga boot device.
> As it needed only GPU driving the display for better user experience.
> Can we use same logic vga_default_device() to claim the interface under root port.
> That will simplify the criteria to claim the interface.

Basically, you need to prevent somebody else from running
DSM_PCI_D3COLD_AUX_POWER_LIMIT concurrently for the given Root Port
and store the information that it has been run already.

Personally, I'd add aux_power_limit to struct acpi_device_power and
I'd use a static mutex in pci_acpi_request_d3cold_aux_power() along
the lines of:

1. Acquire the mutex.
2. If power.aux_power_limit is set for the ACPI companion of pci_dev,
release the mutex and bail out.
3. Evaluate DSM_PCI_D3COLD_AUX_POWER_LIMIT and if it fails, release
the mutex and bail out.
4. Set power.aux_power_limit for the ACPI companion of pci_dev to the
requested value.
5. Release the mutex.

Of course, this would only allow it to be set once per kernel boot, so
in order to handle hibernation properly, the same Aux power limit
would need to be requested again automatically on Root Port restore.