Hi Matthias,
On 2020-08-26 22:10, Matthias Kaehlcke wrote:
> Hi Satya,
>
> On Wed, Aug 26, 2020 at 09:35:15PM +0530, skakit@xxxxxxxxxxxxxx wrote:
> > Hi Matthias,
> >
> > On 2020-08-25 22:08, Matthias Kaehlcke wrote:
> > > On Tue, Aug 25, 2020 at 06:42:28PM +0530, skakit@xxxxxxxxxxxxxx wrote:
> > > > 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.
> > >
> > > As far as I understand the above isn't entirely correct. HW flow control
> > > is not
> > > disabled when the RX FIFO is full, rather as part of HW flow control the
> > > hardware deasserts RTS when the FIFO is full. Software flow control
> > > isn't really
> > > relevant here, since it doesn't use RTS/CTS but uses transmission of
> > > special
> > > codes (XON/XOFF) over TX/RX.
> >
> > Here by Software flow control i meant, we can control the flow from
> > SW(explained below).
>
> Better don't use a term that already has well defined meaning in a
> given context when you refer to something else.
>
Okay.
> > >
> > > > > 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>;
> > >
> > > just noticed this: in the sleep config all pins are in GPIO config (see
> > > "arm64: dts: sc7180: Add wakeup support over UART RX" from this series)
> > > and by default they are inputs, hence the drive-strength here is
> > > pointless
> > > IIUC.
> > >
> >
> > CTS and RX are inputs to the HOST whereas RTS and TX are outputs. We
> > have
> > added drive-strength for output pins only as they are driven by
> > UART(please
> > correct me if wrong).
>
> True, RTS and TX are outputs in UART mode, however in sleep mode the
> pins
> are (currently) configured as GPIOs and inputs (again, see "arm64: dts:
> sc7180: Add wakeup support over UART RX" of this series), hence the
> drive-strength attribute does nothing. If needed/preferred you can
> configure
> the pins as outputs and specify the desired state instead of using
> pulls,
> in that case specifying the drive strength can be useful.
>
Ok, will remove the drive-strength from sleep state.
> > > > > > + 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.
> > >
> > > I'm not sure this is an absolute truth. I think there may be cases where
> > > the driver has to increase its drive strength..
> > >
> > > > 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).
> > >
> > > I disgress. I'm pretty sure that you want RTS to be low to make sure
> > > that
> > > the BT SoC can wake up the system by sending whatever data it has to
> > > send.
> > > It won't do that if RTS is high (e.g. because that's its floating state
> > > at that time). I just tried configuring a pull-up (also a known
> > > non-ambiguous state), and Bluetooth wakeup doesn't work with that,
> > > supposedly because the BT SoC/UART will wait for its CTS signal to be
> > > low.
> > >
> >
> > yes, you are right, we are keeping RTS low to make sure that BT SoC
> > can
> > wakeup the system by sending bytes.
> > My intention here was to explain below case from your comment:
> >
> > > > > 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.
> >
> > 1. By default our HW flow is enabled(since we are configuring
> > pull-down on
> > RTS),and BT SoC can send data anytime.
> > 2. But there is a SW mechanism where we can control the flow from
> > software.
> > In that case what ever is configured to UART_MANUAL_RFR(READY or
> > NOT_READY)
> > will override the dtsi pinconf pull and the RTS/RFR line is pulled
> > high when
> > HOST is not ready(while debugging the wake up issue we came across
> > this).
>
> This is generally correct while the system is running, but (with the
> current
> pinconf) not when the system is suspended IIUC. When the system is in
> suspend
> the function of the UART pins is changed to GPIO, hence the UART ceases
> to
> control RTS.
>
> Otherwise how do you explain that wakeup stops working when you
> configure
> a pull-up instead of a pull-down? According to your comment the UART
> should
> be driving the RTS depending on its readyness.
>
True, I was explaining about UART mode(active case) only, in reply to
your
previous comment:
> > > > > 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.
> > > > >
So, we can keep pull-down in Active case and in sleep state it is
mandatory
to keep pull-down.
