Re: [PATCH 00/10] Add PCIe Bandwidth Controller

[Ilpo Järvinen]

+ thermal people.

On Mon, 11 Sep 2023, Krishna Chaitanya Chundru wrote:
> On 9/4/2023 4:46 PM, Ilpo Järvinen wrote:
> > On Mon, 4 Sep 2023, Krishna Chaitanya Chundru wrote:
> > > On 8/17/2023 5:46 PM, Ilpo Järvinen wrote:
> > > > 
> > > > This series adds PCIe bandwidth controller (bwctrl) and associated PCIe
> > > > cooling driver to the thermal core side for limiting PCIe link speed
> > > > due to thermal reasons. PCIe bandwidth controller is a PCI express bus
> > > > port service driver. A cooling device is created for each port the
> > > > service driver finds if they support changing speeds.
> > > I see we had support for only link speed changes here but we need to add
> > > support for
> > > 
> > > link width change also as bandwidth notification from PCIe supports both
> > > link
> > > speed and link width.
> > Hi,
> > 
> > Thanks for the comment. In case you mean that the changes in Link Width
> > should be reported correctly, they already are since the sysfs interface
> > reads them directly from LNKSTA register.
> > 
> > Or did you perhaps mean that Bandwidth Controller should support also
> > changing Link Width? If this is the case I don't know how it can be
> > realized so a pointer on how it can be achieved would be appreciated.
> 
> I didn't have any idea on how thermal framework works.
> 
> But as we are adding bandwidth controller support we need to add support for
> width change also, may be we are not using this now, but we may need it in the
> future.
> 
> We had similar use case based on the bandwidth requirement on devices like
> WLAN, the client try to reduce or increase the link speed and link width.
> 
> So in the bandwidth controller driver we can add support for link width also.
> So any client can easily use the driver to change link speed or width or both
> to reduce the power consumption.
> 
> Adding link width support should be similar to how you added the link speed
> supported.
> 
> Please correct me if I misunderstood something here.

Hi,

Okay, thanks for the clarification. So the point is to plan for adding 
support for Link Width later and currently only support throttling Link 
Speed. In any case, the Link Width control seems to be controlled using 
a different approach (Link Width change does not require Link Retraining).

I don't know either how such 2 dimensioned throttling (Link Speed and 
Link Width) is supposed to be realized using the thermal/cooling device 
interface which only provides a single integer as the current state. That 
is, whether to provide a single cooling device (with a single integer 
exposed to userspace) or separate cooling device for each dimension?

Perhaps thermal people could provide some insight on this? Is there some 
precedent I could take look at?

-- 
 i.