linux-kernel - Testing the thermal genetlink API

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Message-Id: <20220228110351.20518-1-nicolas.cavallari@green-communications.fr>
Date:   Mon, 28 Feb 2022 12:03:50 +0100
From:   Nicolas Cavallari <nicolas.cavallari@...en-communications.fr>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Amit Kucheria <amitk@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Testing the thermal genetlink API

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.

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.

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