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From:   Marijn Suijten <marijns95@gmail.com>
To:     linux-bluetooth@vger.kernel.org
Cc:     Marijn Suijten <marijns95@gmail.com>,
        Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
        Marek Czerski <ma.czerski@gmail.com>
Subject: [PATCH BlueZ] audio/avrcp: Determine Absolute Volume support from feature category 2
Date:   Thu, 22 Apr 2021 21:22:53 +0200
Message-Id: <20210422192253.553048-1-marijns95@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
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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
---

Hi Luiz, Marek,

It's been quite a while since our last mail contact.  As mentioned
Android simply reports a too low version for its CT despite setting
category 2 for absolute volume support.  Using this feature instead of
the version solves being unable to synchronize volume, is that okay with
you?

- Marijn

 profiles/audio/avrcp.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/profiles/audio/avrcp.c b/profiles/audio/avrcp.c
index 05dd791de..bacd1aeb4 100644
--- a/profiles/audio/avrcp.c
+++ b/profiles/audio/avrcp.c
@@ -4136,13 +4136,16 @@ static void target_init(struct avrcp *session)
 				(1 << AVRCP_EVENT_TRACK_REACHED_END) |
 				(1 << AVRCP_EVENT_SETTINGS_CHANGED);

+	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)
--
2.31.1