Re: [PATCH v4 3/3] pci: intel: Add sysfs attributes to configure pcie link

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Oct 31, 2019 at 12:31:44AM +0100, Rafael J. Wysocki wrote:
> On Wed, Oct 30, 2019 at 11:14 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> >
> > [+cc Heiner, Rajat]
> >
> > On Tue, Oct 29, 2019 at 05:31:18PM +0800, Dilip Kota wrote:
> > > On 10/22/2019 8:59 PM, Bjorn Helgaas wrote:
> > > > [+cc Rafael, linux-pm, beginning of discussion at
> > > > https://lore.kernel.org/r/d8574605f8e70f41ce1e88ccfb56b63c8f85e4df.1571638827.git.eswara.kota@linux.intel.com]
> > > >
> > > > On Tue, Oct 22, 2019 at 05:27:38PM +0800, Dilip Kota wrote:
> > > > > On 10/22/2019 1:18 AM, Bjorn Helgaas wrote:
> > > > > > On Mon, Oct 21, 2019 at 02:38:50PM +0100, Andrew Murray wrote:
> > > > > > > On Mon, Oct 21, 2019 at 02:39:20PM +0800, Dilip Kota wrote:
> > > > > > > > PCIe RC driver on Intel Gateway SoCs have a requirement
> > > > > > > > of changing link width and speed on the fly.
> > > > > > Please add more details about why this is needed.  Since you're adding
> > > > > > sysfs files, it sounds like it's not actually the *driver* that needs
> > > > > > this; it's something in userspace?
> > > > > We have use cases to change the link speed and width on the fly.
> > > > > One is EMI check and other is power saving.  Some battery backed
> > > > > applications have to switch PCIe link from higher GEN to GEN1 and
> > > > > width to x1. During the cases like external power supply got
> > > > > disconnected or broken. Once external power supply is connected then
> > > > > switch PCIe link to higher GEN and width.
> > > > That sounds plausible, but of course nothing there is specific to the
> > > > Intel Gateway, so we should implement this generically so it would
> > > > work on all hardware.
> > > Agree.
> > > >
> > > > I'm not sure what the interface should look like -- should it be a
> > > > low-level interface as you propose where userspace would have to
> > > > identify each link of interest, or is there some system-wide
> > > > power/performance knob that could tune all links?  Cc'd Rafael and
> > > > linux-pm in case they have ideas.
> > >
> > > To my knowledge sysfs is the appropriate way to go.
> > > If there are any other best possible knobs, will be helpful.
> >
> > I agree sysfs is the right place for it; my question was whether we
> > should have files like:
> >
> >   /sys/.../0000:00:1f.3/pcie_speed
> >   /sys/.../0000:00:1f.3/pcie_width
> >
> > as I think this patch would add (BTW, please include sample paths like
> > the above in the commit log), or whether there should be a more global
> > thing that would affect all the links in the system.
> >
> > I think the low-level files like you propose would be better because
> > one might want to tune link performance differently for different
> > types of devices and workloads.
> >
> > We also have to decide if these files should be associated with the
> > device at the upstream or downstream end of the link.  For ASPM, the
> > current proposal [1] has the files at the downstream end on the theory
> > that the GPU, NIC, NVMe device, etc is the user-recognizable one.
> > Also, neither ASPM nor link speed/width make any sense unless there
> > *is* a device at the downstream end, so putting them there
> > automatically makes them visible only when they're useful.
> >
> > Rafael had some concerns about the proposed ASPM interface [2], but I
> > don't know what they are yet.
> 
> I was talking about the existing ASPM interface in sysfs.  The new one
> I still have to review, but I'm kind of wondering what about people
> who used the old one?  Would it be supported going forward?

The old one interface was enabled by CONFIG_PCIEASPM_DEBUG.  Red Hat
doesn't enable that.  Ubuntu does.  I *thought* we heard from a
Canonical person who said they didn't have any tools that used it, but
I can't find that now.  I don't know about SUSE.

So the idea was to drop it on the theory that nobody is using it.
Possibly that's too aggressive.

Bjorn