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

[Dilip Kota]

On 10/31/2019 6:14 AM, Bjorn Helgaas 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@xxxxxxxxxxxxxxx]
> > >
> > > 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.
Sure, i will add them.
> 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.

This patch places the speed and width in the host controller directory.
/sys/.../xxx.pcie/pcie_speed
/sys/.../xxx.pcie/pcie_width

I agree with you partially, because i am having couple of points making 
me to
keep speed and width change entries in controller directory:

-- For changing the speed/width with device node, software ends up 
traversing to the controller
 from the device and do the operations.
-- Change speed and width are performed at controller level,
-- Keeping speed and width in controller gives a perspective (to the 
user) of changing
them only once irrespective of no. of devices.
-- For speed and link change in Synopsys PCIe controller, specific 
registers need to be configured.
ÂÂ This prevents or complicates adding the speed and width change 
functionality in pci-sysfs or pci framework.

Regards,
Dilip