Re: [PATCH 02/12] PCI/ACPI: Add PERST# Assertion Delay _DSM method

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Apr 02, 2025 at 01:06:42PM +0200, Rafael J. Wysocki wrote:
> On Tue, Apr 1, 2025 at 5:36 PM Anshuman Gupta <anshuman.gupta@intel.com> wrote:
> >
> > Implement _DSM Method 11 as per PCI firmware specs
> > section 4.6.11 Rev 3.3.

> > +int pci_acpi_add_perst_assertion_delay(struct pci_dev *dev, u32 delay_us)
> > +{
> > +       union acpi_object in_obj = {
> > +               .integer.type = ACPI_TYPE_INTEGER,
> > +               .integer.value = delay_us,
> > +       };
> > +
> > +       union acpi_object *out_obj;
> > +       acpi_handle handle;
> > +       int result, ret = -EINVAL;
> > +
> > +       if (!dev || !ACPI_HANDLE(&dev->dev))
> > +               return -EINVAL;
> > +
> > +       handle = ACPI_HANDLE(&dev->dev);
> > +
> > +       out_obj = acpi_evaluate_dsm_typed(handle, &pci_acpi_dsm_guid, 4,
> 
> This is something I haven't noticed in the previous patch, but also
> applies to it.
> 
> Why is rev 4 of the interface hard-coded here?

Thanks for asking this because it's related to the whole _DSM revision
question that I don't understand.

If we didn't use rev 4 here, what should we use?  The PCI Firmware
spec, r3.3, sec 4.6.11, documents this interface and says "lowest
valid Revision ID value is 4", so that's the source of the 4.

My argument is that the spec documents rev 4, the kernel code was
tested with rev 4, so what would be the benefit of using a different
revision here?

> > +                                         DSM_PCI_PERST_ASSERTION_DELAY,
> > +                                         &in_obj, ACPI_TYPE_INTEGER);