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Date:   Wed, 19 Aug 2020 19:18:54 +0530
From:   skakit@codeaurora.org
To:     Matthias Kaehlcke <mka@chromium.org>
Cc:     Bjorn Andersson <bjorn.andersson@linaro.org>,
        gregkh@linuxfoundation.org, Andy Gross <agross@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        linux-arm-msm@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, akashast@codeaurora.org,
        rojay@codeaurora.org, msavaliy@qti.qualcomm.com
Subject: Re: [PATCH V2 2/3] arm64: dts: qcom: sc7180: Add sleep pin ctrl for
 BT uart
In-Reply-To: <20200817233341.GE2995789@google.com>
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 <1595563082-2353-3-git-send-email-skakit@codeaurora.org>
 <20200817180158.GD2995789@google.com> <20200817233341.GE2995789@google.com>
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Hi Matthias,

Thanks for reviewing the patches.

On 2020-08-18 05:03, Matthias Kaehlcke wrote:
> On Mon, Aug 17, 2020 at 11:01:58AM -0700, Matthias Kaehlcke wrote:
>> On Fri, Jul 24, 2020 at 09:28:01AM +0530, satya priya wrote:
>> > Add sleep pin ctrl for BT uart, and also change the bias
>> > configuration to match Bluetooth module.
>> >
>> > Signed-off-by: satya priya <skakit@codeaurora.org>
>> > ---
>> > Changes in V2:
>> >  - This patch adds sleep state for BT UART. Newly added in V2.
>> >
>> >  arch/arm64/boot/dts/qcom/sc7180-idp.dts | 42 ++++++++++++++++++++++++++++-----
>> >  1 file changed, 36 insertions(+), 6 deletions(-)
>> >
>> > diff --git a/arch/arm64/boot/dts/qcom/sc7180-idp.dts b/arch/arm64/boot/dts/qcom/sc7180-idp.dts
>> > index 26cc491..bc919f2 100644
>> > --- a/arch/arm64/boot/dts/qcom/sc7180-idp.dts
>> > +++ b/arch/arm64/boot/dts/qcom/sc7180-idp.dts
>> > @@ -469,20 +469,50 @@
>> >
>> >  &qup_uart3_default {
>> >  	pinconf-cts {
>> > -		/*
>> > -		 * Configure a pull-down on 38 (CTS) to match the pull of
>> > -		 * the Bluetooth module.
>> > -		 */
>> > +		/* Configure no pull on 38 (CTS) to match Bluetooth module */
>> 
>> Has the pull from the Bluetooth module been removed or did the 
>> previous config
>> incorrectly claim that the Bluetooth module has a pull-down?
>> 
>> >  		pins = "gpio38";
>> > +		bias-disable;
>> > +	};
>> > +
>> > +	pinconf-rts {
>> > +		/* We'll drive 39 (RTS), so configure pull-down */
>> > +		pins = "gpio39";
>> > +		drive-strength = <2>;
>> >  		bias-pull-down;
>> > +	};
>> > +
>> > +	pinconf-tx {
>> > +		/* We'll drive 40 (TX), so no pull */
>> 
>> The rationales for RTS and TX contradict each other. According to the 
>> comment
>> the reason to configure a pull-down on RTS is that it is driven by the 
>> host.
>> Then for TX the reason to configure no pull is that it is driven by 
>> the host.
>> 
>> Please make sure the comments *really* describe the rationale, 
>> otherwise they
>> are just confusing.
> 
> Ok, let's try to reason about the configurations.
> 
> I didn't find the datasheet for the WCN3991, but my understanding is 
> that
> it is an evolution of the WCN3998, so probably the states of the UART 
> pins
> are the same (signal names from the BT chip perspective):
> 
>      active   reset
> CTS    NP      PD
> RTS    NP      PD
> RX     NP      PU
> TX     NP      PD
> 
> Since this patch changes the DT let's use the signal names from the 
> host side
> in the following.
> 
>> RTS: NP => PD
> 
> I can see that this could make sense, a floating pin could indicate
> the Bluetooth controller that the host is ready to receive data, when 
> it is
> not.
> 
>> CTS: PD => NP
> 
> From a signalling perspective this should be no problem, since the 
> WCN399x
> has a pull-down on its RTS signal in reset, and otherwise will drive 
> it.
> IIUC there should be no power leakage without a pull, so I think this
> should be ok.
> 

With CTS having no-pull, we are not seeing any power leakages.

>> TX: +output-high
> 
> IIUC this only has an impact when the pin is in GPIO mode, i.e. in the 
> sleep
> config. If that's correct, does it even make sense to specify it in the 
> default
> config?
> 
> Besides that, what is the reason for this change? I was told in another 
> forum
> that Qualcomm found this to fix problems at UART initialization and 
> wakeup,
> without really understanding why. That's not great.
> 

"output-high" was present in IDP dts since Bring-up, we've validated on 
the latest code-base and see that "output-high" is not required, will 
remove it.

> I'm no expert in this area, but my guess is that forcing the TX signal 
> to high
> in certain states is needed to not have it floating (no pull is 
> configured),
> which could generate garbage on the Bluetooth RX side. But is it really
> necessary to actively drive it to high? Wouldn't it be enough to 
> configure a
> pull-up when it isn't actively driven (i.e. in sleep mode)?
> 
> In a quick test wakeup from Bluetooth worked when configuring a pull-up 
> only in
> sleep mode. Could you test this on your side or provide a rationale why 
> TX needs
> to be actively driven to high?
> 

We have tested by keeping pull-up for TX in sleep state(removed 
output-high) and wakeup is working fine with Bluetooth. Will remove the 
output-high from both default and sleep states.

> Thanks
> 
> Matthias

Thanks,
Satya Priya