linux-kernel - Re: [PATCH v3 1/8] thermal: core: Add mechanism for connecting trips with driver data

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Message-ID: <CAJZ5v0iH+qf6eBuZASPKyA6rT8O6FiA7516MiYYUx6Uc+wR4Ow@mail.gmail.com>
Date:   Thu, 3 Aug 2023 21:58:32 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Michal Wilczynski <michal.wilczynski@...el.com>,
        Zhang Rui <rui.zhang@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: [PATCH v3 1/8] thermal: core: Add mechanism for connecting trips
 with driver data

On Thu, Aug 3, 2023 at 6:20 PM Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
>
> On 03/08/2023 16:15, Rafael J. Wysocki wrote:
> > On Thu, Aug 3, 2023 at 3:06 PM Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
> >>
> >> On 02/08/2023 18:48, Rafael J. Wysocki wrote:
> >>
> >> [ ... ]
> >>
> >>>> Let me check if I can do something on top of your series to move it in
> >>>> the ACPI driver.
> >>>
> >>> It doesn't need to be on top of my series, so if you have an idea,
> >>> please just let me know what it is.
> >>>
> >>> It can't be entirely in the ACPI driver AFAICS, though, because
> >>> trips[i] need to be modified on updates and they belong to the core.
> >>> Hence, the driver needs some help from the core to get to them.  It
> >>> can be something like "this is my trip tag and please give me the
> >>> address of the trip matching it" or similar, but it is needed, because
> >>> the driver has to assume that the trip indices used by it initially
> >>> may change.
> >>
> >> May be I'm missing something but driver_ref does not seems to be used
> >> except when assigning it, no?
> >
> > It is used on the other side.  That is, the value assigned to the trip
> > field in it is accessed via trip_ref in the driver.
> >
> > The idea is that the driver puts a pointer to its local struct
> > thermal_trip_ref into a struct thermal_trip and the core stores the
> > address of that struct thermal_trip in there, which allows the driver
> > to access the struct thermal_trip via its local struct
> > thermal_trip_ref going forward.
> >
> > Admittedly, this is somewhat convoluted.
> >
> > I have an alternative approach in the works, just for illustration
> > purposes if nothing else, but I have encountered a problem that I
> > would like to ask you about.
> >
> > Namely, zone disabling is not particularly useful for preventing the
> > zone from being used while the trips are updated, because it has side
> > effects.  First, it triggers __thermal_zone_device_update() and a
> > netlink message every time the mode changes, which can be kind of
> > overcome.
>
> Right
>
> > But second, if the mode is "disabled", it does not actually
> > prevent things like __thermal_zone_get_trip() from running and the
> > zone lock is the only thing that can be used for that AFAICS.
>  >
> > So by "disabling" a thermal zone, did you mean changing its mode to
> > "disabled" or something else?
>
> Yes, that is what I meant.
>
> May be the initial proposal by updating the thermal trips pointer can
> solve that [1]

No, it can't.  An existing trips[] table cannot be replaced with a new
one with different trip indices, because those indices are already in
use.  And if the indices are the same, there's no reason to replace
trips.

> IMO we can assume the trip point changes are very rare (if any), so
> rebuilding a new trip array and update the thermal zone with the pointer
> may solve the situation.
>
> The routine does a copy of the trips array, so it can reorder it without
> impacting the array passed as a parameter. And it can take the lock.

The driver can take a lock as well.  Forbidding drivers to use the
zone lock is an artificial limitation without technical merit IMV.

> We just have to constraint the update function to invalidate arrays with
> a number of trip points different from the one initially passed when
> creating the thermal zone.
>
> Alternatively, we can be smarter in the ACPI driver and update the
> corresponding temperature+hysteresis trip point by using the
> thermal_zone_set_trip() function.

I don't see why this would make any difference.

> [1]
> https://lore.kernel.org/all/20230525140135.3589917-5-daniel.lezcano@linaro.org/