linux-kernel - Re: Testing the thermal genetlink API

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Date:   Mon, 28 Feb 2022 13:21:30 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Nicolas Cavallari <nicolas.cavallari@...en-communications.fr>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Amit Kucheria <amitk@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Testing the thermal genetlink API


Hi Nicolas,

thanks for using the netlink and giving those feedbacks even you are 
unhappy with them.

On 28/02/2022 12:03, Nicolas Cavallari wrote:
> I've played a bit with the thermal netlink interface and it wasn't pleasant:
> 
> 1. The way attributes are used is painful.  Instead of using arrays of
>     nested structs-like, it flattens them into a big nested attr where
>     you have to guess when an entry starts and when it ends.
>     libnl provides no helper for this case:
> 
>     [{nla_type=TZ|F_NESTED},
> 	[{nla_type=TZ_ID}, 1]
> 	[{nla_type=TZ_NAME}, "name1"]
> 	[{nla_type=TZ_ID}, 2]
> 	[{nla_type=TZ_NAME}, "name2"]
> 	[{nla_type=TZ_ID}, 3]
> 	[{nla_type=TZ_NAME}, "name3"]
> 	[{nla_type=TZ_ID}, 4]
> 	[{nla_type=TZ_NAME}, "name4"]
>     ]
> 
> 2. The genl_cmd types are not unique between multicast events and
>     command replies.  If you send genl_cmd=3 (CMD_TZ_GET_TEMP) and you
>     get a genl_cmd=3 reply, you cannot know if it is a CMD_TZ_GET_TEMP
>     response or a EVENT_TZ_DISABLE because both have genl_cmd=3, but
>     completely different semantics.
> 3. The API is heavy.  Getting the complete information about all thermal
>     zones requires 1 + 6 * thermal_zones netlink requests, each of them
>     only returning few information.  You need most of them to merely
>     translate the event's TZ_ID/TZ_TRIP_ID/CDEV_ID to names.

That is part of the discovery and it should happen only once when you 
get the thermal information.

> 4. THERMAL_GENL_CMD_TZ_GET_TRIP cause an oops if the thermal zone driver
>     does not have a get_trip_hyst callback.
>     This concerns all drivers, short of two.  A patch follows.

Great, thanks for the fix.

> For the record, I couldn't find any open source program using this API.
> It's also not enabled in all distributions.

The netlink support is very recent. A library has been posted [1] and 
hopefully it can helps you to get rid of all the complexity.

Thanks for testing

   -- D.

[1] 
https://lore.kernel.org/all/20220218125334.995447-1-daniel.lezcano@linaro.org/


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