[PATCH BlueZ v2] audio/avrcp: Determine Absolute Volume support from feature category 2

From: Marijn Suijten <marijn.suijten@somainline.org>
To: linux-bluetooth@vger.kernel.org
Cc: Marijn Suijten <marijn.suijten@somainline.org>,
	Pauli Virtanen <pav@iki.fi>,
	Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
	Marek Czerski <ma.czerski@gmail.com>
Subject: [PATCH BlueZ v2] audio/avrcp: Determine Absolute Volume support from feature category 2
Date: Tue, 19 Oct 2021 11:16:48 +0200	[thread overview]
Message-ID: <20211019091648.120910-1-marijn.suijten@somainline.org> (raw)

The AVRCP spec (1.6.2) does not mention anything about a version
requirement for Absolute Volume, despite this feature only existing
since spec version 1.4.  Android reports a version of 1.3 [1] for its
"AVRCP remote" (CT) service and mentions in the comment above it itself
relies on feature bits rather than the exposed version.  As it stands
BlueZ requires at least version 1.4 making it unable to communicate
absolute volume levels with even the most recent Android phones running
Fluoride (have not checked the version on Gabeldorsche).

The spec states that supporting SetAbsoluteVolume and
EVENT_VOLUME_CHANGED are mandatory when feature level 2 is declared,
excluded otherwise.  This feature bit is set on Android and, when used
by this patch, allows for successfully communicating volume back and
forth despite the version theoretically being too low.

[1]: https://android.googlesource.com/platform/system/bt/+/android-11.0.0_r28/bta/av/bta_av_main.cc#761
---

Changes since v1:
- Use block comment intead of single-line comment.

 profiles/audio/avrcp.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/profiles/audio/avrcp.c b/profiles/audio/avrcp.c
index d3c9cb795..e530eeab4 100644
--- a/profiles/audio/avrcp.c
+++ b/profiles/audio/avrcp.c
@@ -4175,13 +4175,17 @@ static void target_init(struct avrcp *session)
 				(1 << AVRCP_EVENT_TRACK_REACHED_END) |
 				(1 << AVRCP_EVENT_SETTINGS_CHANGED);
 
+	/* Remote device supports receiving volume notifications */
+	if (target->features & AVRCP_FEATURE_CATEGORY_2)
+		session->supported_events |=
+				(1 << AVRCP_EVENT_VOLUME_CHANGED);
+
 	if (target->version < 0x0104)
 		return;
 
 	session->supported_events |=
 				(1 << AVRCP_EVENT_ADDRESSED_PLAYER_CHANGED) |
-				(1 << AVRCP_EVENT_AVAILABLE_PLAYERS_CHANGED) |
-				(1 << AVRCP_EVENT_VOLUME_CHANGED);
+				(1 << AVRCP_EVENT_AVAILABLE_PLAYERS_CHANGED);
 
 	/* Only check capabilities if controller is not supported */
 	if (session->controller == NULL)
-- 
2.33.1

