Re: [PATCH] PCI/sysfs: Protect driver's D3cold preference from user space

[Lukas Wunner <lukas@wunner.de>]

On Mon, Sep 18, 2023 at 08:28:51AM -0500, Mario Limonciello wrote:
> On 9/18/2023 08:24, Lukas Wunner wrote:
> > On Mon, Sep 18, 2023 at 08:14:21AM -0500, Mario Limonciello wrote:
> > > What's the history behind why userspace is allowed to opt a device out of
> > > D3cold in the first place?
> > > 
> > > It feels like it should have been a debugging only thing to me.
> > 
> > That's a fair question.
> > 
> > Apparently the default for d3cold_allowed was originally "false"
> > and user space could opt in to D3cold.  Then commit 4f9c1397e2e8
> > ("PCI/PM: Enable D3/D3cold by default for most devices") changed
> > the default to "true".  That was 11 years ago.
> > 
> > I agree that today this should all work automatically and a
> > user space option to disable D3cold on a per-device basis only
> > really makes sense as a debugging aid, hence belongs in debugfs.
> > 
> 
> Thanks.  Then perhaps as part of moving it to debugfs it makes sense to
> simplify the logic.

d3cold_allowed is documented in Documentation/ABI/testing/sysfs-bus-pci,
so it's user space ABI which we're not allowed to break.  We'd have to
declare it deprecated, emit a warning when it's used and slowly phase
it out over the years.

Until then, the $SUBJECT_PATCH probably still makes sense to reinstate
the behavior we had until 2016...

Thanks,

Lukas