Re: [PATCH] thermal/core: Introduce user trip points

Rafael J. Wysocki
2024-07-02T11:18:06Z

On Tue, Jul 2, 2024 at 1:03 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Tue, Jul 2, 2024 at 12:56 PM Daniel Lezcano
> <daniel.lezcano@linaro.org> wrote:
> >
> > On 02/07/2024 12:22, Rafael J. Wysocki wrote:
> > > On Tue, Jul 2, 2024 at 11:29 AM Daniel Lezcano
> > > <daniel.lezcano@linaro.org> wrote:
> > >>
> > >> On 01/07/2024 18:26, Rob Herring wrote:
> > >>> On Thu, Jun 27, 2024 at 10:54:50AM +0200, Daniel Lezcano wrote:
> > >>>> Currently the thermal framework has 4 trip point types:
> > >>>>
> > >>>> - active : basically for fans (or anything requiring energy to cool
> > >>>> down)
> > >>>>
> > >>>> - passive : a performance limiter
> > >>>>
> > >>>> - hot : for a last action before reaching critical
> > >>>>
> > >>>> - critical : a without return threshold leading to a system shutdown
> > >>>>
> > >>>> A thermal zone monitors the temperature regarding these trip
> > >>>> points. The old way to do that is actively polling the temperature
> > >>>> which is very bad for embedded systems, especially mobile and it is
> > >>>> even worse today as we can have more than fifty thermal zones. The
> > >>>> modern way is to rely on the driver to send an interrupt when the trip
> > >>>> points are crossed, so the system can sleep while the temperature
> > >>>> monitoring is offloaded to a dedicated hardware.
> > >>>>
> > >>>> However, the thermal aspect is also managed from userspace to protect
> > >>>> the user, especially tracking down the skin temperature sensor. The
> > >>>> logic is more complex than what we found in the kernel because it
> > >>>> needs multiple sources indicating the thermal situation of the entire
> > >>>> system.
> > >>>>
> > >>>> For this reason it needs to setup trip points at different levels in
> > >>>> order to get informed about what is going on with some thermal zones
> > >>>> when running some specific application.
> > >>>>
> > >>>> For instance, the skin temperature must be limited to 43°C on a long
> > >>>> run but can go to 48°C for 10 minutes, or 60°C for 1 minute.
> > >>>>
> > >>>> The thermal engine must then rely on trip points to monitor those
> > >>>> temperatures. Unfortunately, today there is only 'active' and
> > >>>> 'passive' trip points which has a specific meaning for the kernel, not
> > >>>> the userspace. That leads to hacks in different platforms for mobile
> > >>>> and embedded systems where 'active' trip points are used to send
> > >>>> notification to the userspace. This is obviously not right because
> > >>>> these trip are handled by the kernel.
> > >>>>
> > >>>> This patch introduces the 'user' trip point type where its semantic is
> > >>>> simple: do nothing at the kernel level, just send a notification to
> > >>>> the user space.
> > >>>
> > >>> Sounds like OS behavior/policy though I guess the existing ones kind are
> > >>> too. Maybe we should have defined *what* action to take and then the OS
> > >>> could decide whether what actions to handle vs. pass it up a level.
> > >>
> > >> Right
> > >>
> > >>> Why can't userspace just ask to be notified at a trip point it
> > >>> defines?
> > >>
> > >> Yes I think it is possible to create a netlink message to create a trip
> > >> point which will return a trip id.
> > >>
> > >> Rafael what do you think ?
> > >
> > > Trips cannot be created on the fly ATM.
> > >
> > > What can be done is to create trips that are invalid to start with and
> > > then set their temperature via sysfs. This has been done already for
> > > quite a while AFAICS.
> >
> > Yes, I remember that.
> >
> > I would like to avoid introducing more weirdness in the thermal
> > framework which deserve a clear ABI.
> >
> > What is missing to create new trip points on the fly ?
>
> A different data structure to store them (essentially, a list instead
> of an array).
>
> I doubt it's worth the hassle.
>
> What's wrong with the current approach mentioned above? It will need
> to be supported going forward anyway.

BTW, there are two different concepts that seem to be mixed here.

One of them is a "trigger" that will cause a netlink message to be
sent to user space when a given temperature level is crossed (either
way) and nothing more. This in principle can be added to any thermal
zone (even tripless) and should be possible to implement as a separate
mechanism independent of trip points.

The other one is a pair of trip points that can be set "around" the
current zone temperature so that the .set_trips() callback uses them
to program interrupts to trigger when one of them is crossed. This at
least requires the thermal zone to provide a .set_trips() callback, so
it depends on the driver registering the thermal zone. Arguably, the
driver in question can reserve a pair of "trip slots" in the trip
table passed to the zone registration function.