linux-kernel - Re: [PATCH 2/6] thermal: of: Export non-devres helper to register/unregister thermal zone

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Message-ID: <Z5pkUNNvsWPjRQvy@mai.linaro.org>
Date: Wed, 29 Jan 2025 18:24:32 +0100
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: Claudiu <claudiu.beznea@...on.dev>
Cc: rafael@...nel.org, rui.zhang@...el.com, lukasz.luba@....com,
	robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org,
	geert+renesas@...der.be, magnus.damm@...il.com,
	mturquette@...libre.com, sboyd@...nel.org, p.zabel@...gutronix.de,
	ulf.hansson@...aro.org, linux-pm@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-renesas-soc@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-clk@...r.kernel.org,
	Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
Subject: Re: [PATCH 2/6] thermal: of: Export non-devres helper to
 register/unregister thermal zone

Hi Claudiu,

On Fri, Jan 03, 2025 at 06:38:01PM +0200, Claudiu wrote:
> From: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
> 
> On the Renesas RZ/G3S (and other Renesas SoCs, e.g., RZ/G2{L, LC, UL}),
> clocks are managed through PM domains. These PM domains, registered on
> behalf of the clock controller driver, are configured with
> GENPD_FLAG_PM_CLK. In most of the Renesas drivers used by RZ SoCs, the
> clocks are enabled/disabled using runtime PM APIs.
> 
> During probe, devices are attached to the PM domain controlling their
> clocks. Similarly, during removal, devices are detached from the PM domain.
> 
> The detachment call stack is as follows:
> 
> device_driver_detach() ->
>   device_release_driver_internal() ->
>     __device_release_driver() ->
>       device_remove() ->
>         platform_remove() ->
> 	  dev_pm_domain_detach()
> 
> In the upcoming Renesas RZ/G3S thermal driver, the
> struct thermal_zone_device_ops::change_mode API is implemented to
> start/stop the thermal sensor unit. Register settings are updated within
> the change_mode API.
> 
> In case devres helpers are used for thermal zone register/unregister the
> struct thermal_zone_device_ops::change_mode API is invoked when the
> driver is unbound. The identified call stack is as follows:
> 
> device_driver_detach() ->
>   device_release_driver_internal() ->
>     device_unbind_cleanup() ->
>       devres_release_all() ->
>         devm_thermal_of_zone_release() ->
> 	  thermal_zone_device_disable() ->
> 	    thermal_zone_device_set_mode() ->
> 	      rzg3s_thermal_change_mode()
> 
> The device_unbind_cleanup() function is called after the thermal device is
> detached from the PM domain (via dev_pm_domain_detach()).
> 
> The rzg3s_thermal_change_mode() implementation calls
> pm_runtime_resume_and_get()/pm_runtime_put_autosuspend() before/after
> accessing the registers. However, during the unbind scenario, the
> devm_thermal_of_zone_release() is invoked after dev_pm_domain_detach().
> Consequently, the clocks are not enabled, as the device is removed from
> the PM domain at this time, leading to an Asynchronous SError Interrupt.
> The system cannot be used after this.

I've been through the driver before responding to this change. What is the
benefit of powering down / up (or clock off / on) the thermal sensor when
reading the temperature ?

I can understand for disable / enable but I don't get for the classic usage
where a governor will be reading the temperature regularly.

Would the IP need some cycles to capture the temperature accurately after the
clock is enabled ?

> Add thermal_of_zone_register()/thermal_of_zone_unregister(). These will
> be used in the upcomming RZ/G3S thermal driver.
> 
> Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>