linux-kernel - Re: [PATCH V2 2/5] tty: serial: fsl_lpuart: clear UARTCTRL_LOOPS in lpuart32

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Date:   Wed, 23 Nov 2022 12:09:20 +0100
From:   Michael Walle <michael@...le.cc>
To:     Sherry Sun <sherry.sun@....com>
Cc:     gregkh@...uxfoundation.org, jirislaby@...nel.org,
        jingchang.lu@...escale.com, tomonori.sakita@...d.co.jp,
        atsushi.nemoto@...d.co.jp, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org, dl-linux-imx <linux-imx@....com>
Subject: Re: [PATCH V2 2/5] tty: serial: fsl_lpuart: clear UARTCTRL_LOOPS in
 lpuart32_shutdown()

Am 2022-11-23 11:58, schrieb Sherry Sun:
>> -----Original Message-----
>> From: Michael Walle <michael@...le.cc>
>> Sent: 2022年11月23日 18:34
>> To: Sherry Sun <sherry.sun@....com>
>> Cc: gregkh@...uxfoundation.org; jirislaby@...nel.org;
>> jingchang.lu@...escale.com; tomonori.sakita@...d.co.jp;
>> atsushi.nemoto@...d.co.jp; linux-serial@...r.kernel.org; linux-
>> kernel@...r.kernel.org; dl-linux-imx <linux-imx@....com>
>> Subject: Re: [PATCH V2 2/5] tty: serial: fsl_lpuart: clear 
>> UARTCTRL_LOOPS in
>> lpuart32_shutdown()
>> 
>> Am 2022-11-10 09:17, schrieb Sherry Sun:
>> > UARTCTRL_LOOPS bit is set in lpuart32_set_mctrl() for loopback mode,
>> > but nowhere clear this bit, it should be cleared when closing the uart
>> > port to avoid the loopback mode been enabled by default when reopening
>> > the uart.
>> 
>> It's cleared in set_mctrl(). What is the expectation from the serial 
>> core here?
>> 
> 
> Hi Michael,
> 
> If we call .set_mctrl(TIOCM_LOOP), the UARTCTRL_LOOPS will be set.
> Then when we call .shutdown(), serial core won't call .set_mctrl() to
> clear it, so the UARTCTRL_LOOPS need to be cleared here.
> Per my understanding, .shutdown() should clean up all the uart flags,
> as the transmitter and receiver will been disabled, we will
> re-configure all the settings needed when re-open the port.

Two things,
(1) should the loopback be cleared on a newly opened serial device?
(2) as mentioned in my other reply, this can also be handled in
     the startup. Eg. the startup can clear the loopback flag.
     (together with possible hardware events).

I'm not that deep into the serial core, thus my question about
the expectations from the serial core. I guess the answer to
(1) is yes, but better to ask.

-michael