Re: [PATCH v2 2/8] serial: sh-sci: Check if TX data was written to device in .tx_empty()

[Jiri Slaby <jirislaby@kernel.org>]

Hi,

On 08. 11. 24, 13:19, Claudiu Beznea wrote:
> On 08.11.2024 12:57, Jiri Slaby wrote:
>> On 08. 11. 24, 11:05, Claudiu wrote:
...
>>> --- a/drivers/tty/serial/sh-sci.c
>>> +++ b/drivers/tty/serial/sh-sci.c
>>> @@ -157,6 +157,7 @@ struct sci_port {
>>>          bool has_rtscts;
>>>        bool autorts;
>>> +    bool first_time_tx;
>>
>> This is a misnomer. It suggests to be set only during the first TX.
> 
> I chose this naming as this was the scenario I discovered it didn't work.
> Reproducible though these steps:
> 
> 1/ open the serial device (w/o running any TX/RX)
> 2/ call tx_empty()
> 
> What
>> about ::did_tx, ::performed_tx, ::transmitted, or alike?
> 
> I have nothing against any of these. Can you please let me know if you have
> a preferred one?

No, you choose, or invent even better one :). Or let AI do it for you.

>>> @@ -885,6 +887,7 @@ static void sci_transmit_chars(struct uart_port *port)
>>>            }
>>>              sci_serial_out(port, SCxTDR, c);
>>> +        s->first_time_tx = true;
>>>              port->icount.tx++;
>>>        } while (--count > 0);
>>> @@ -1241,6 +1244,8 @@ static void sci_dma_tx_complete(void *arg)
>>>        if (kfifo_len(&tport->xmit_fifo) < WAKEUP_CHARS)
>>>            uart_write_wakeup(port);
>>>    +    s->first_time_tx = true;
>>
>> This is too late IMO. The first in-flight dma won't be accounted in
>> sci_tx_empty(). From DMA submit up to now.
> 
> If it's in-flight we can't determine it's status anyway with one variable.
> We can set this variable later but it wouldn't tell the truth as the TX
> might be in progress anyway or may have been finished?
> 
> The hardware might help with this though the TEND bit. According to the HW
> manual, the TEND bit has the following meaning:
> 
> 0: Transmission is in the waiting state or in progress.
> 1: Transmission is completed.
> 
> But the problem, from my point of view, is that the 0 has double meaning.
> 
> I noticed the tx_empty() is called in kernel multiple times before
> declaring TX is empty or not. E.g., uart_suspend_port() call it 3 times,
> uart_wait_until_sent() call it in a while () look with a timeout. There is
> the uart_ioctl() which calls it though uart_get_lsr_info() only one time
> but I presumed the user space might implement the same multiple trials
> approach before declaring it empty.
> 
> Because of this I considered it wouldn't be harmful for the scenario you
> described "The first in-flight dma won't be accounted in sci_tx_empty()"
> as the user may try again later to check the status. For this reason I also
> chose to have no extra locking around this variable.

What about the below?

>>> @@ -2076,6 +2081,10 @@ static unsigned int sci_tx_empty(struct uart_port
>>> *port)
>>>    {
>>>        unsigned short status = sci_serial_in(port, SCxSR);
>>>        unsigned short in_tx_fifo = sci_txfill(port);
>>> +    struct sci_port *s = to_sci_port(port);
>>> +
>>> +    if (!s->first_time_tx)
>>> +        return TIOCSER_TEMT;
>>
>> So perhaps check if there is a TX DMA running here too?

This ^^^? Like dmaengine_tx_status()?

>>
>>>          return (status & SCxSR_TEND(port)) && !in_tx_fifo ? TIOCSER_TEMT
>>> : 0;
>>>    }
>>
>> thanks,
-- 
js
suse labs