Re: [PATCH V3 2/3] arm64: dts: qcom: sc7180: Add sleep pin ctrl for BT uart
From: skakit
Date: Tue Aug 25 2020 - 09:12:38 EST
On 2020-08-21 22:52, Matthias Kaehlcke wrote:
On Thu, Aug 20, 2020 at 07:21:06PM +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>
Reviewed-by: Akash Asthana <akashast@xxxxxxxxxxxxxx>
---
Changes in V2:
- This patch adds sleep state for BT UART. Newly added in V2.
Changes in V3:
- Remove "output-high" for TX from both sleep and default states
as it is not required. Configure pull-up for TX in sleep state.
arch/arm64/boot/dts/qcom/sc7180-idp.dts | 54
+++++++++++++++++++++++++++------
1 file changed, 45 insertions(+), 9 deletions(-)
diff --git a/arch/arm64/boot/dts/qcom/sc7180-idp.dts
b/arch/arm64/boot/dts/qcom/sc7180-idp.dts
index d8b5507..806f626 100644
--- a/arch/arm64/boot/dts/qcom/sc7180-idp.dts
+++ b/arch/arm64/boot/dts/qcom/sc7180-idp.dts
@@ -473,20 +473,20 @@
&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 */
pins = "gpio38";
- bias-pull-down;
- output-high;
+ bias-disable;
I think it should be ok in functional terms, but I don't like the
rationale
and also doubt the change is really needed.
If the pull is removed to match the Bluetooth module, then that sounds
as
if the signal was floating on the the BT side, which I think is not the
case.
Yes, according to the datasheet there is no pull when the BT controller
is
active, but then it drives the signal actively to either high or low.
There
seems to be no merit in 'matching' the Bluetooth side in this case, if
the
signal was really floating on the BT side we would definitely not want
this.
In a reply to v2 you said:
Recently on cherokee we worked with BT team and came to an agreement
to
keep no-pull from our side in order to not conflict with their pull in
any state.
What are these conflicting pull states?
The WCN3998 datasheet has a pull-down on RTS (WCN3998 side) in reset
and
boot mode, and no pull in active mode. In reset and boot mode the host
config with a pull down would match, and no pull in active mode doesn't
conflict with the pull-down on the host UART. My understanding is that
the pinconf pulls are weak pulls, so as soon as the BT chip drives its
RTS the pull on the host side shouldn't matter.
yes, I agree with you, the pinconf pulls are weak. As this is driven by
BT SoC (pull on HOST side shouldn't matter), we are not mentioning any
bias configuration from our side and simply putting it as no-pull, just
to not conflict in any case. It seems that the rationale mentioned is a
bit confusing i will change it to clearly specify why we are configuring
no-pull.
Is this change actually related with wakeup support? I have the
impression
that multiple things are conflated in this patch. If some of the
changes
are just fixing/improving other things they should be in a separate
patch,
which could be part of this series, otherwise it's really hard to
distinguish between the pieces that are actually relevant for wakeup
and
the rest.
Independently of whether the changes are done in a single or multiple
patches, the commit log should include details on why the changes are
necessary, especially when there are not explantatory comments in the
DT/code itself (e.g. the removal of 'output-high', which seems correct
to me, but no reason is given why it is done).
This change is not related to wakeup support, I will make it a separate
patch, will also mention the details in commit text.
};
pinconf-rts {
- /* We'll drive 39 (RTS), so no pull */
+ /*
+ * Configure pull-down on 39 (RTS). This is needed to avoid a
+ * floating pin which could mislead Bluetooth controller
+ * with UART RFR state (READY/NOT_READY).
+ */
pins = "gpio39";
drive-strength = <2>;
- bias-disable;
+ bias-pull-down;
};
[copy of my comment on v2]
I'm a bit at a loss here, about two things:
RTS is an output pin controlled by the UART. IIUC if the UART port is
active
and hardware flow control is enabled the RTS signal is either driven to
high
or low, but not floating.
Yes, RTS is either driven high or low. HW flow control is always enabled
and only turned off when RX FIFO is full. Whereas SW flow control is
controlled by upper layers(serial core), also it can be enabled/disabled
from host by calling set_mctrl.
Now lets assume I'm wrong with the above and RTS can be floating. We
only want
the BT SoC to send data when the host UART is ready to receive them,
right?
RTS is an active low signal, hence by configuring it as a pull-down the
BT
SoC can send data regardless of whether the host UART actually asserts
RTS,
so the host UART may not be ready to receive it. I would argue that if
there
is really such a thing as a floating RTS signal then it should have a
pull-up,
to prevent the BT SoC from sending data at any time.
I'm not an expert in UART communication and pinconf, so it could be
that I
got something wrong, but as of now it seems to me that no pull is the
correct
config for RTS.
pinconf-tx {
@@ -494,7 +494,43 @@
pins = "gpio40";
drive-strength = <2>;
bias-disable;
- output-high;
+ };
+
+ pinconf-rx {
+ /*
+ * Configure a pull-up on 41 (RX). This is needed to avoid
+ * garbage data when the TX pin of the Bluetooth module is
+ * in tri-state (module powered off or not driving the
+ * signal yet).
+ */
+ pins = "gpio41";
+ bias-pull-up;
+ };
+};
+
+&qup_uart3_sleep {
+ pinconf-cts {
+ /* Configure no-pull on 38 (CTS) to match Bluetooth module */
+ pins = "gpio38";
+ bias-disable;
+ };
+
+ pinconf-rts {
+ /*
+ * Configure pull-down on 39 (RTS). This is needed to avoid a
+ * floating pin which could mislead Bluetooth controller
+ * with UART RFR state (READY/NOT_READY).
+ */
+ pins = "gpio39";
+ drive-strength = <2>;
+ bias-pull-down;
+ };
I don't know all the details, but I have the impression that this is
the
relevant pull change for wakeup. From the title of the series I derive
that the UART RX pin is used for signalling wakeup. A pull-down on RTS
indicates the BT controller that it can always send data to wake up the
host.
I think RTS in default mode should remain with no-pull (the UART is
driving
the signal), and then change it to pull-down in sleep mode.
As I understand from your previous comment, pinconf pulls are weak and
cannot override the pull of controller. Although pull down is
configured, data will be received only if host controller is ready to
accept it. So, we want to put RTS in pull-down state(known state)
instead of leaving it in ambiguous state i.e, no-pull(high/low).
+
+ pinconf-tx {
+ /* Configure pull-up on 40 (TX) when it isn't actively driven */
nit: just say '... on TX ...', the GPIO number isn't really interesting
and can
easily be determined by looking at 'pins' if needed . Applicable to all
comments
involving pins.
Okay.
+ pins = "gpio40";
+ drive-strength = <2>;
+ bias-pull-up;
This makes sense to me.
Thanks,
Satya Priya