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Subject: Re: [PATCHv2] Bluetooth: quirk disabling LE Read Transmit Power
From:   Marcel Holtmann <marcel@holtmann.org>
In-Reply-To: <20211117124717.12352-1-redecorating@protonmail.com>
Date:   Wed, 17 Nov 2021 20:01:17 +0100
Cc:     regressions@leemhuis.info, danielwinkler@google.com,
        gargaditya08@live.com, gregkh@linuxfoundation.org,
        Johan Hedberg <johan.hedberg@intel.com>,
        linux-bluetooth@vger.kernel.org, linux-kernel@vger.kernel.org,
        Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
        regressions@lists.linux.dev, sonnysasaka@chromium.org
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To:     Orlando Chamberlain <redecorating@protonmail.com>
Precedence: bulk

Hi Orlando,

>> So if this just affects two macs, why can't the fix be realized as a
>> quirk that is only enabled on those two systems? Or are they impossible
>> to detect clearly via DMI data or something like that?
> 
> I think we should be able to quirk based off the acpi _CID "apple-uart-blth"
> or _HID "BCM2E7C". Marcel suggested quirking based of the acpi table here
> https://lore.kernel.org/linux-bluetooth/1D2217A9-EA73-4D93-8D0B-5BC2718D4788@holtmann.org/
> 
> This would catch some unaffected Macs, but they don't support the LE Read
> Transmit Power command anyway (the affected macs were released after it
> was added to the Bluetooth spec, while the unaffected Macs were released
> before it was added to the spec, and thus don't support it).
> 
> I'm not sure how to go about applying a quirk based off this, there are
> quirks in drivers/bluetooth/hci_bcm.c (no_early_set_baudrate and
> drive_rts_on_open), but they don't seem to be based off acpi ids.
> 
> It might be simpler to make it ignore the Unknown Command error, like
> in this patch https://lore.kernel.org/linux-bluetooth/CABBYNZLjSfcG_KqTEbL6NOSvHhA5-b1t_S=3FQP4=GwW21kuzg@mail.gmail.com/
> however that only applies on bluetooth-next and needed the status it
> checks for to be -56, not 0x01.

so we abstain from try-and-error sending of commands. The Bluetooth spec
has a list of supported commands that a host can query for a reason. This
is really broken behavior of the controller and needs to be pointed out as
such.

The question is just how we quirk it.

Regards

Marcel