linux-kernel - [PATCH 3/3] ahci: Document the loss of hotplug by new LPM policy

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Date:   Tue, 24 May 2022 12:05:08 -0500
From:   Mario Limonciello <mario.limonciello@....com>
To:     Damien Le Moal <damien.lemoal@...nsource.wdc.com>
CC:     "open list:LIBATA SUBSYSTEM (Serial and Parallel ATA drivers)" 
        <linux-ide@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        <hdegoede@...hat.com>,
        Mario Limonciello <mario.limonciello@....com>
Subject: [PATCH 3/3] ahci: Document the loss of hotplug by new LPM policy

Per AHCI spec v1.3.1, "7.3 Native Hot Plug Support", once LPM is
enabled hotplug support needs to be disabled.

The LPM code always followed this and disabled the port when no
drives were connected, but as more machines will be exposed to
this code it might be an unexpected behavior to some users.

Add a note to parameter documentation to explain the new behavior.

Link: https://bugs.launchpad.net/bugs/1971576
Signed-off-by: Mario Limonciello <mario.limonciello@....com>
---
 Documentation/admin-guide/kernel-parameters.txt | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9e6bd212004d..4dcd9a3ba4a5 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -279,6 +279,13 @@
 			3 => Medium power with Device Initiated PM enabled
 			4 => Minimum power
 
+			NOTE: Enabling LPM when no drive is connected will disable
+			the port which means hotplug will not work.
+
+			If hotplug is an important use case, this can be modified
+			at runtime by changing
+			/sys/module/ahci/parameters/lpm_policy
+
 	ALSA		[HW,ALSA]
 			See Documentation/sound/alsa-configuration.rst
 
-- 
2.34.1