linux-kernel - Re: [PATCH v5 2/3] tty: serial: use uart_port

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Date:   Tue, 22 Nov 2022 09:18:48 +0100
From:   Michael Walle <michael@...le.cc>
To:     Jiri Slaby <jirislaby@...nel.org>
Cc:     afaerber@...e.de, alexandre.belloni@...tlin.com,
        claudiu.beznea@...rochip.com, festevam@...il.com,
        gregkh@...uxfoundation.org, ilpo.jarvinen@...ux.intel.com,
        kernel@...gutronix.de, linux-imx@....com,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
        liviu.dudau@....com, lorenzo.pieralisi@....com, mani@...nel.org,
        nicolas.ferre@...rochip.com, richard.genoud@...il.com,
        s.hauer@...gutronix.de, shawnguo@...nel.org, sudeep.holla@....com,
        tklauser@...tanz.ch, vz@...ia.com
Subject: Re: [PATCH v5 2/3] tty: serial: use uart_port_tx() helper

Am 2022-11-22 09:09, schrieb Michael Walle:
> Hi,
> 
> Am 2022-11-22 08:02, schrieb Jiri Slaby:
>> On 21. 11. 22, 21:27, Michael Walle wrote:
>>> This will break serial output for the userspace on my board
>>> (arch/arm/boot/dts/lan966x-kontron-kswitch-d10-mmt*dts). The 
>>> uart_port_tx()
>>> helper will call __port->ops->stop_tx(__port) if 
>>> uart_circ_chars_pending()
>>> returns 0. But the code above, doesn't do that. In fact, removing the
>>> stop_tx() call in the helper macro, will fix the console output.
>>> 
>>> Any ideas how to fix that?
>> 
>> Hm, so ATMEL_US_TXRDY is removed from tx_done_mask in stop_tx, but not
>> added back in start_tx. So the tx interrupt is never handled (the tx
>> tasklet is not scheduled to send the queue chars) in
>> atmel_handle_transmit().
>> 
>> Any chance, the below fixes it?
>> 
>> diff --git a/drivers/tty/serial/atmel_serial.c
>> b/drivers/tty/serial/atmel_serial.c
>> index 11bf2466390e..395370e0c77b 100644
>> --- a/drivers/tty/serial/atmel_serial.c
>> +++ b/drivers/tty/serial/atmel_serial.c
>> @@ -596,6 +596,8 @@ static void atmel_start_tx(struct uart_port *port)
>>                 /* re-enable PDC transmit */
>>                 atmel_uart_writel(port, ATMEL_PDC_PTCR, 
>> ATMEL_PDC_TXTEN);
>> 
>> +       atmel_port->tx_done_mask |= ATMEL_US_TXRDY;
>> +
>>         /* Enable interrupts */
>>         atmel_uart_writel(port, ATMEL_US_IER, 
>> atmel_port->tx_done_mask);
>> 
>> 
>> thanks,
> 
> Unfortunately, that doesn't help. Btw, some characters are transmitted:
> 
> 
> [    6.219356] Key type dns_resolver registered
> [    6.223679] Registering SWP/SWPB emulation handler
> [    6.247530] Loading compiled-in X.509 certificates
> [    6.288467] Freeing unused kernel image (initmem) memory: 1024K
> [    6.297789] Run /init as init process
> WbSOROSOSOSOSOStarting linuxptp system clock synchronization: O
> 
> -michael

But you made me look at atmel_stop_tx() and there is this:

        /*
	* Disable the transmitter.
	* This is mandatory when DMA is used, otherwise the DMA buffer
	* is fully transmitted.
	*/
	atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXDIS);

Removing that write, will also fix the problem. Could it be, that
the transmit is still active (via DMA) but the driver will call
tx_stop() and then stop the transmission in the background?

-michael