Re: [PATCH V2 4/8] tty: serial: qcom_geni_serial: Add interconnect support

From: Matthias Kaehlcke <mka@chromium.org>
To: Akash Asthana <akashast@codeaurora.org>
Cc: gregkh@linuxfoundation.org, agross@kernel.org,
	bjorn.andersson@linaro.org, wsa@the-dreams.de,
	broonie@kernel.org, mark.rutland@arm.com, robh+dt@kernel.org,
	linux-i2c@vger.kernel.org, linux-spi@vger.kernel.org,
	devicetree@vger.kernel.org, swboyd@chromium.org,
	mgautam@codeaurora.org, linux-arm-msm@vger.kernel.org,
	linux-serial@vger.kernel.org, dianders@chromium.org,
	evgreen@chromium.org
Subject: Re: [PATCH V2 4/8] tty: serial: qcom_geni_serial: Add interconnect support
Date: Tue, 17 Mar 2020 12:08:04 -0700	[thread overview]
Message-ID: <20200317190804.GS144492@google.com> (raw)
In-Reply-To: <e9293de6-004f-6005-8cb6-66f28c080ebe@codeaurora.org>

On Tue, Mar 17, 2020 at 05:18:34PM +0530, Akash Asthana wrote:
> Hi Matthias,
> 
> On 3/14/2020 2:58 AM, Matthias Kaehlcke wrote:
> > Hi Akash,
> > 
> > On Fri, Mar 13, 2020 at 06:42:10PM +0530, Akash Asthana wrote:
> > > Get the interconnect paths for Uart based Serial Engine device
> > > and vote according to the baud rate requirement of the driver.
> > > 
> > > Signed-off-by: Akash Asthana <akashast@codeaurora.org>
> > > ---
> > > Changes in V2:
> > >   - As per Bjorn's comment, removed se == NULL check from geni_serial_icc_get
> > >   - As per Bjorn's comment, removed code to set se->icc_path* to NULL in failure
> > >   - As per Bjorn's comment, introduced and using devm_of_icc_get API for getting
> > >     path handle
> > >   - As per Matthias comment, added error handling for icc_set_bw call
> > > 
> > >   drivers/tty/serial/qcom_geni_serial.c | 69 +++++++++++++++++++++++++++++++++--
> > >   1 file changed, 65 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
> > > index 272bae0..c8ad7e9 100644
> > > --- a/drivers/tty/serial/qcom_geni_serial.c
> > > +++ b/drivers/tty/serial/qcom_geni_serial.c
> > >
> > > ...
> > >
> > >   static int qcom_geni_serial_request_port(struct uart_port *uport)
> > >   {
> > >   	struct platform_device *pdev = to_platform_device(uport->dev);
> > > @@ -962,6 +975,7 @@ static void qcom_geni_serial_set_termios(struct uart_port *uport,
> > >   	struct qcom_geni_serial_port *port = to_dev_port(uport, uport);
> > >   	unsigned long clk_rate;
> > >   	u32 ver, sampling_rate;
> > > +	int ret;
> > >   	qcom_geni_serial_stop_rx(uport);
> > >   	/* baud rate */
> > > @@ -983,6 +997,18 @@ static void qcom_geni_serial_set_termios(struct uart_port *uport,
> > >   	ser_clk_cfg = SER_CLK_EN;
> > >   	ser_clk_cfg |= clk_div << CLK_DIV_SHFT;
> > > +	/*
> > > +	 * Put BW vote only on CPU path as driver supports FIFO mode only.
> > > +	 * Assume peak_bw as twice of avg_bw.
> > > +	 */
> > > +	port->se.avg_bw_cpu = Bps_to_icc(baud);
> > > +	port->se.peak_bw_cpu = Bps_to_icc(2 * baud);
> > > +	ret = icc_set_bw(port->se.icc_path_cpu_to_geni, port->se.avg_bw_cpu,
> > > +			port->se.peak_bw_cpu);
> > > +	if (ret)
> > > +		dev_err(uport->dev, "%s: ICC BW voting failed for cpu\n",
> > > +			__func__);
> > Should this return an error? The port might not operate properly if the ICC
> > bandwidth couldn't be configured
> 
> This is void function we can't return error from here. I guess it would be
> somewhat okay if BW voting failed for CPU path but clk_set_rate failure is
> more serious which is called from this function, I don't think it can be
> move to somewhere else.

ok, I missed that _set_termios() is void.

> > >   static const struct uart_ops qcom_geni_console_pops = {
> > > @@ -1308,6 +1358,17 @@ static int qcom_geni_serial_probe(struct platform_device *pdev)
> > >   	port->rx_fifo_depth = DEF_FIFO_DEPTH_WORDS;
> > >   	port->tx_fifo_width = DEF_FIFO_WIDTH_BITS;
> > > +	ret = geni_serial_icc_get(&port->se);
> > > +	if (ret)
> > > +		return ret;
> > > +	/* Set the bus quota to a reasonable value */
> > > +	port->se.avg_bw_core = console ? Bps_to_icc(1000) :
> > > +		Bps_to_icc(CORE_2X_50_MHZ);
> > Why different settings for console vs. non-console?
> 
> QUP FW runs on core clock. To support higher throughput we want FW to run at
> higher speed.
> 
> Since Console operate at 115200bps and BT operate at 3.2Mbps baud. We are
> voting higher on core for BT usecase.
> 
> These value are recommended from HW team.

IIUC none of the values you mention are set in stone. 115200bps seems to be a
'standard' value for the serial console, but it could be a different baudrate.
I guess you are referring to Qualcomm Bluetooth controllers, which are only one
of many things that could be connected to the port. And what happens when a
QCA BT controller is connected to a non-geni/QCA port, which doesn't know about
its 'requirements'? The answer is that both the BT controller and the serial
console configure the baudrate they need, hence using different values in
_probe() is pointless.

Unsurprisingly one of the first things the QCA BT driver does is to configure
the baudrate. It typically starts with a lower ('init') speed, and then switches
to the higher ('operational') baudrate:

https://elixir.bootlin.com/linux/v5.5.8/source/drivers/bluetooth/hci_qca.c#L1256