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

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Heiner for ASPM conversation]

On Tue, Oct 29, 2019 at 11:42:53AM +0100, Rafael J. Wysocki wrote:
> On Tue, Oct 22, 2019 at 2:59 PM Bjorn Helgaas <helgaas@kernel.org> 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.
> >
> > 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.
> 
> Frankly, I need some time to think about this and, in case you are
> wondering about whether or not it has been discussed with me already,
> it hasn't.
> 
> At this point I can only say that since we have an ASPM interface,
> which IMO is not fantastic, it may be good to come up with a common
> link management interface.

The ASPM interface hasn't been merged yet, so if you have better
ideas, now is the time.  That one is definitely very low-level, partly
because the first use case is working around defects in a specific
device.

Some sort of unification of link management does sound like a good
idea.

Bjorn