Re: [PATCH RFT 1/6] serial: sh-sci: Check if TX data was written to device in .tx_empty()

[Claudiu Beznea <claudiu.beznea@tuxon.dev>]

Hi, Geert,

On 19.12.2024 11:46, Geert Uytterhoeven wrote:
> Hi Claudiu,
> 
> On Wed, Dec 4, 2024 at 4:58 PM Claudiu <claudiu.beznea@tuxon.dev> wrote:
>> From: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
>>
>> On the Renesas RZ/G3S, when doing suspend to RAM, the uart_suspend_port()
>> is called. The uart_suspend_port() calls 3 times the
>> struct uart_port::ops::tx_empty() before shutting down the port.
>>
>> According to the documentation, the struct uart_port::ops::tx_empty()
>> API tests whether the transmitter FIFO and shifter for the port is
>> empty.
>>
>> The Renesas RZ/G3S SCIFA IP reports the number of data units stored in the
>> transmit FIFO through the FDR (FIFO Data Count Register). The data units
>> in the FIFOs are written in the shift register and transmitted from there.
>> The TEND bit in the Serial Status Register reports if the data was
>> transmitted from the shift register.
>>
>> In the previous code, in the tx_empty() API implemented by the sh-sci
>> driver, it is considered that the TX is empty if the hardware reports the
>> TEND bit set and the number of data units in the FIFO is zero.
>>
>> 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.
>>
>> It has been noticed that when opening the serial device w/o using it and
>> then switch to a power saving mode, the tx_empty() call in the
>> uart_port_suspend() function fails, leading to the "Unable to drain
>> transmitter" message being printed on the console. This is because the
>> TEND=0 if nothing has been transmitted and the FIFOs are empty. As the
>> TEND=0 has double meaning (waiting state, in progress) we can't
>> determined the scenario described above.
>>
>> Add a software workaround for this. This sets a variable if any data has
>> been sent on the serial console (when using PIO) or if the DMA callback has
>> been called (meaning something has been transmitted). In the tx_empty()
>> API the status of the DMA transaction is also checked and if it is
>> completed or in progress the code falls back in checking the hardware
>> registers instead of relying on the software variable.
>>
>> Fixes: 73a19e4c0301 ("serial: sh-sci: Add DMA support.")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
> Thanks for your patch, which is now commit 7cc0e0a43a910524 ("serial:
> sh-sci: Check if TX data was written to device in .tx_empty()") in
> v6.13-rc3.
> 
>> --- a/drivers/tty/serial/sh-sci.c
>> +++ b/drivers/tty/serial/sh-sci.c
>> @@ -885,6 +887,7 @@ static void sci_transmit_chars(struct uart_port *port)
>>                 }
>>
>>                 sci_serial_out(port, SCxTDR, c);
>> +               s->tx_occurred = true;
> And you cannot use the existing port->icount.tx below, as that is not
> reset to zero on sci_startup(), right?

I missed that the driver is incrementing the port->icount.tx . I'm not sure
we can use it though, as it is not reset on sci_startup(), as you pointed out.

Thank you,
Claudiu