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From:   Hans de Goede <hdegoede@redhat.com>
To:     Marcel Holtmann <marcel@holtmann.org>,
        Johan Hedberg <johan.hedberg@gmail.com>
Cc:     Hans de Goede <hdegoede@redhat.com>,
        Hui Wang <hui.wang@canonical.com>,
        linux-bluetooth@vger.kernel.org
Subject: [PATCH 5.12 regression fix 0/1] Bluetooth: btusb: Revert "Fix the autosuspend enable and disable"
Date:   Thu, 18 Feb 2021 13:37:27 +0100
Message-Id: <20210218123728.17067-1-hdegoede@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Hi All,

From the commit msg:

"""
drivers/usb/core/hub.c: usb_new_device() contains the following:

        /* By default, forbid autosuspend for all devices.  It will be
         * allowed for hubs during binding.
         */
        usb_disable_autosuspend(udev);

So for anything which is not a hub, such as btusb devices, autosuspend is
disabled by default and we MUST call usb_enable_autosuspend(udev) to
enable it.

This means that the "Fix the autosuspend enable and disable" commit,
which drops the usb_enable_autosuspend() call when the enable_autosuspend
module option is true, is completely wrong, revert it.
"""

Hui, I guess that what you were seeing is caused by:
/lib/udev/hwdb.d/60-autosuspend-chromiumos.hwdb

Which enables autosuspend on a bunch of USB devices based on VID:PID,
overruling the kernel defaults. This is done to get better power-consumption
with devices where it is known that it is safe to do this.
 
I guess that that the device you were testing this with is on that list.
So the proper fix would be to edit that file and remove your VID:PID from it.

Hui, also next time please try to Cc the original author of the code you
are modifying. A simple "git blame drivers/bluetooth/btusb.c" would have
found you commit eff2d68ca738 ("Bluetooth: btusb: Add a Kconfig option to
enable USB autosuspend by default") and then you could have added me to
the Cc and I could have nacked the patch before it got merged.

I happen to spot this this time since I was looking into some other
btusb issue. But if I had not spotted this, this would have caused
a significant power-consumption regression on many laptops.

Btusb might not look like a big consumer, but if it does not autosuspend
it often is the only USB device not autosuspending, keeping the XHCI
controller awake, which in turn is keeping a whole power-plane awake on
what once used to be the southbridge. At least on Skylake era hw this
could lead to an extra idle powerconsumption of 1W. So a small change
can cause a big impact.

Regards,

Hans