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

[Akash Asthana <akashast@codeaurora.org>]

Hi Matthias,

On 3/18/2020 12:38 AM, Matthias Kaehlcke wrote:
> 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.

Are you refering other UART drivers(not based on geni HW) as 
non-geni/QCA port?

We are not scaling core BW request based on real time need like we are 
doing for other paths(CPU/DDR) instead we are using some fail proof 
value because, FW runs on core clock and core behaves a bit different 
than other NOCs.

We don't have any functional relation which maps actual throughput 
requirement to core frequency need. In the past we faced few latency 
issues because of core slowness (Although it was running much higher 
than actual throughput requirement). To avoid such scenario we are using 
recommend value from HW team. These fix value can support SE drivers 
operating at their max possible speed(4Mbps in case of non-console).

I agree that 115200bps seems to be a 'standard' value for the serial 
console, but it could be a different baudrate.

We are voting 1000 in case of console because it  has low power mode 
use-case in android, where voting CORE_2X_50_MHZ can be reported as a 
power issue.

Actually we wanted to vote 960 for console but that is not possible with 
current ICC design where the minimum value is 1000bps.  So any way core 
is running at 50 MHz as 1000 crosses the threshold for 19.2 MHz (960)

only with console.

regards,

Akash

> 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

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