Re: [PATCH V2 2/3] arm64: dts: qcom: sc7180: Add sleep pin ctrl for BT uart

[skakit]

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@xxxxxxxxxxxxxx>
> > > ---
> > > 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