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From:   Kai-Heng Feng <kai.heng.feng@canonical.com>
Date:   Sat, 9 Jan 2021 00:25:28 +0800
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Subject: Multiple MODALIAS= in uevent file confuses userspace
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        lennart@poettering.net,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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Commit 8765c5ba19490 ("ACPI / scan: Rework modalias creation when
"compatible" is present") creates two modaliases for certain ACPI
devices. However userspace (systemd-udevd in this case) assumes uevent
file doesn't have duplicated keys, so two "MODALIAS=" breaks the
assumption.

Based on the assumption, systemd-udevd internally uses hashmap to
store each line of uevent file, so the second modalias always replaces
the first modalias.

My attempt [1] is to add a new key, "MODALIAS1" for the second
modalias. This brings up the question of whether each key in uevent
file is unique. If it's no unique, this may break may userspace.

[1] https://github.com/systemd/systemd/pull/18163

Kai-Heng