linux-kernel - Re: [PATCH 1/2] vga_switcheroo: add power support for windows 10 machines.

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Date:	Mon, 14 Mar 2016 12:23:57 +0200
From:	Mika Westerberg <mika.westerberg@...ux.intel.com>
To:	Dave Airlie <airlied@...il.com>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Linux PCI <linux-pci@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] vga_switcheroo: add power support for windows 10
 machines.

On Mon, Mar 14, 2016 at 07:47:39PM +1000, Dave Airlie wrote:
> >
> >> -     if (pcie_port_runtime_suspend_allowed(dev))
> >> +     if (pcie_port_runtime_suspend_allowed(dev)) {
> >> +             pm_runtime_allow(&dev->dev);
> >
> > PCI drivers typically have left this decision up to the userspace. I'm
> > wondering whether it is good idea to deviate from that here? Of course
> > this allows immediate power savings but could potentially cause problems
> > as well.
> >
> 
> No distro has ever shipped userspace to do this, I really think this
> is a bad design.
> We have wasted countless watts of power on this stupid idea that people will
> run powertop, only a few people in the world run powertop, lots of
> people use Linux.

That is a fair point.

I do not have anything against calling pm_runtime_allow() here. In fact
we already do the same in Intel LPSS drivers. I just wanted to bring
that up.

Rafael, what do you think?

If we anyway are going to add cut-off date to enable runtime PM we
should expect that the hardware is also capable of doing so (and if not
we can always blacklist the exceptions).

> The kernel should power stuff down not wait for the user to run powertop,
> At least for the GPU it's in the area of 8W of power, and I've got the
> GPU drivers doing this themselves,
> 
> I could have the GPU driver call runtime allow for it's host bridge I suppose,
> if we insist on the userspace cares, but I'd prefer not doing so.
> 
> > I think we need to add corresponding call to pm_runtime_forbid() in
> > pcie_portdrv_remove().
> 
> Yes most likely.

BTW, I can add both calls to the next version of PCIe runtime PM patches
if you are OK with that, and all agree this is a good idea.