Re: [PATCH 2/9] serial: sh-sci: Check if TX data was written to device in .tx_empty()

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Nov 06, 2024 at 02:01:11PM +0200, Claudiu 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).
> 
> Fixes: 73a19e4c0301 ("serial: sh-sci: Add DMA support.")
> Cc: stable@vger.kernel.org
> Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
> ---
>  drivers/tty/serial/sh-sci.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
> index df523c744423..8e2d534401fa 100644
> --- a/drivers/tty/serial/sh-sci.c
> +++ b/drivers/tty/serial/sh-sci.c
> @@ -153,6 +153,7 @@ struct sci_port {
>  	int				rx_trigger;
>  	struct timer_list		rx_fifo_timer;
>  	int				rx_fifo_timeout;
> +	atomic_t			first_time_tx;

Don't use an atomic variable for an informational thing like this, it is
racy and doesn't work properly.  Either use a real lock (because you
care about the locking stuff here), or just use a boolean and live with
any potential races.



>  	u16				hscif_tot;
>  
>  	bool has_rtscts;
> @@ -850,6 +851,7 @@ static void sci_transmit_chars(struct uart_port *port)
>  {
>  	struct tty_port *tport = &port->state->port;
>  	unsigned int stopped = uart_tx_stopped(port);
> +	struct sci_port *s = to_sci_port(port);
>  	unsigned short status;
>  	unsigned short ctrl;
>  	int count;
> @@ -885,6 +887,7 @@ static void sci_transmit_chars(struct uart_port *port)
>  		}
>  
>  		sci_serial_out(port, SCxTDR, c);
> +		atomic_set(&s->first_time_tx, 1);
>  
>  		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);
>  
> +	atomic_set(&s->first_time_tx, 1);
> +
>  	if (!kfifo_is_empty(&tport->xmit_fifo)) {
>  		s->cookie_tx = 0;
>  		schedule_work(&s->work_tx);
> @@ -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 (!atomic_read(&s->first_time_tx))
> +		return TIOCSER_TEMT;

See, what happens here if the value changes right after you check it?
Being an atomic doesn't mean anything :(

thanks,

greg k-h