Re: [PATCH 1/2] PCI: Add a helper to check Power Resource Requirements _PR3 existence

[Peter Wu <peter@lekensteyn.nl>]

On Tue, Aug 27, 2019 at 05:13:21PM -0500, Bjorn Helgaas wrote:
> [+cc Peter, Mika, Dave]
> 
> https://lore.kernel.org/r/20190827134756.10807-1-kai.heng.feng@canonical.com
> 
> On Wed, Aug 28, 2019 at 12:58:28AM +0800, Kai-Heng Feng wrote:
> > at 23:25, Takashi Iwai <tiwai@suse.de> wrote:
> > > On Tue, 27 Aug 2019 15:47:55 +0200,
> > > Kai-Heng Feng wrote:
> > > > A driver may want to know the existence of _PR3, to choose different
> > > > runtime suspend behavior. A user will be add in next patch.
> > > > 
> > > > This is mostly the same as nouveau_pr3_present().
> > > 
> > > Then it'd be nice to clean up the nouveau part, too?
> > 
> > nouveau_pr3_present() may call pci_d3cold_disable(), and my intention is to
> > only check the presence of _PR3 (i.e. a dGPU) without touching anything.
> 
> It looks like Peter added that code with 279cf3f23870
> ("drm/nouveau/acpi: use DSM if bridge does not support D3cold").
> 
> I don't understand the larger picture, but it is somewhat surprising
> that nouveau_pr3_present() *looks* like a simple predicate with no
> side-effects, but in fact it disables the use of D3cold in some cases.

The reason for disabling _PR3 from that point on is because mixing the
ACPI firmware code that uses power resources (_PR3) with the legacy
_DSM/_PS0/_PS3 methods to manage power states could break as that
combination is unlikely to be supported nor tested by firmware authors.

If a user sets /sys/bus/pci/devices/.../d3cold_allowed to 0, then the
pci_d3cold_disable call ensures that this action is remembered and
prevents power resources from being used again.

For example, compare this power resource _OFF code:
https://github.com/Lekensteyn/acpi-stuff/blob/b55f6bdb/dsl/Clevo_P651RA/ssdt3.dsl#L454-L471

with this legacy _PS0/_PS3 code:
https://github.com/Lekensteyn/acpi-stuff/blob/b55f6bdb/dsl/Clevo_P651RA/ssdt7.dsl#L113-L142

The power resource code checks the "MSD3" variable to check whether a
transition to OFF is required while the legacy _PS3 checks "DGPS". The
sequence PG00._OFF followed by _DSM (to to change "OPCE") and _PS3 might
trigger some device-specific code twice and could lead to lockups
(infinite loops polling for power state) or worse. I am not sure if I
have ever tested this scenario however.

> If the disable were moved to the caller, Kai-Heng's new interface
> could be used both places.

Moving the pci_d3cold_disable call to the caller looks reasonable to me.
After the first patch gets merged, nouveau could use something like:

    *has_pr3 = pci_pr3_present(pdev);
    if (*has_pr3 && !pdev->bridge_d3) {
        /*
         * ...
         */
        pci_d3cold_disable(pdev);
        *has_pr3 = false;
    }


For the 1/2 patch,
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
-- 
Kind regards,
Peter Wu
https://lekensteyn.nl