Re: [PATCH v2 1/4] dt-bindings: bluetooth: add new wcn3991 compatible to fix bd_addr

From: Rob Herring <robh@kernel.org>
To: Johan Hovold <johan@kernel.org>
Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
	Johan Hovold <johan+linaro@kernel.org>,
	Marcel Holtmann <marcel@holtmann.org>,
	Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	Conor Dooley <conor+dt@kernel.org>,
	Johan Hedberg <johan.hedberg@gmail.com>,
	Matthias Kaehlcke <mka@chromium.org>,
	Doug Anderson <dianders@google.com>,
	Bjorn Andersson <quic_bjorande@quicinc.com>,
	Konrad Dybcio <konrad.dybcio@linaro.org>,
	linux-bluetooth@vger.kernel.org, linux-arm-msm@vger.kernel.org,
	netdev@vger.kernel.org, devicetree@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/4] dt-bindings: bluetooth: add new wcn3991 compatible to fix bd_addr
Date: Mon, 18 Mar 2024 09:48:06 -0500	[thread overview]
Message-ID: <20240318144806.GA3963554-robh@kernel.org> (raw)
In-Reply-To: <Zfg--2_NMPSPTxK-@hovoldconsulting.com>

On Mon, Mar 18, 2024 at 02:17:47PM +0100, Johan Hovold wrote:
> On Mon, Mar 18, 2024 at 03:00:40PM +0200, Dmitry Baryshkov wrote:
> > On Mon, 18 Mar 2024 at 13:09, Johan Hovold <johan+linaro@kernel.org> wrote:
> > >
> > > Several Qualcomm Bluetooth controllers lack persistent storage for the
> > > device address and instead one can be provided by the boot firmware
> > > using the 'local-bd-address' devicetree property.
> > >
> > > The Bluetooth bindings clearly says that the address should be specified
> > > in little-endian order, but due to a long-standing bug in the Qualcomm
> > > driver which reversed the address some bootloaders have been providing
> > > the address in big-endian order instead.
> > >
> > > The only device out there that should be affected by this is the WCN3991
> > > used in some Chromebooks. To maintain backwards compatibility, mark the
> > > current compatible string as deprecated and add a new
> > > 'qcom,wcn3991-bt-bdaddr-le' for firmware which conforms with the
> > > binding.
> 
> > This compatible doesn't describe new hardware kind. As such, I think,
> > the better way would be to continue using qcom,wcn3991-bt compatible
> > string + add some kind of qcom,bt-addr-le property.
> 
> No, you can't handle backwards compatibility by *adding* a property.

But you could add a property for the not broken case. That's a bit odd, 
but so is your compatible.

> I wanted to avoid doing this, but if we have to support Google's broken
> boot firmware for these devices, then this is how it needs to be done.

Don't Chromebooks update everything together. So maybe we don't care in 
this case?

Rob