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:

quoted

On Fri, Jul 24, 2020 at 09:28:01AM +0530, satya priya wrote:

quoted

Add sleep pin ctrl for BT uart, and also change the bias
configuration to match Bluetooth module.

Signed-off-by: satya priya <redacted>
---
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?

quoted

 		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.

quoted

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.

quoted

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.

quoted

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