linux-kernel - Re: [PATCH 1/2] PCI: Add a helper to check Power Resource Requirements

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Message-ID: <20190827221321.GA9987@google.com>
Date:   Tue, 27 Aug 2019 17:13:21 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Kai-Heng Feng <kai.heng.feng@...onical.com>
Cc:     Takashi Iwai <tiwai@...e.de>, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        Peter Wu <peter@...ensteyn.nl>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Dave Airlie <airlied@...hat.com>
Subject: Re: [PATCH 1/2] PCI: Add a helper to check Power Resource
 Requirements _PR3 existence

[+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@...e.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.

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