Re: [PATCH 3/3] tty/serial: at91: fix hardware handshake when DMA is not used

[Cyrille Pitchen]

Hi Richard,

For usart without FIFOs (hence before sama5d2), according to our designers, the
RTS line could only been controlled by an internal PDC signal which doesn't
exist with the DMA controller. Referring to its datasheet, the sam9g35 embeds
DMA controllers. So if you enable the hardware handshaking feature on some
usart, its RTS line won't be monitored at all. The hardware handshaking is
broken on all SoCs using DMA controllers instead of PDC.

With the sama5d2, our latest MPU, we fixed this issue by introducing an
alternative mechanism which relies on 2 thresholds on the RX FIFO. So when
FIFOs are available, those thresholds can be used to control the level of RTS
line.

Indeed I think a better test should be:
if (!atmel_use_pdc_rx(port) && !atmel_use_fifo(port)) {
	dev_info(port->dev, "not enabling hardware flow control because neither the PDC nor the FIFO are available");
	termios->c_cflags &= ~CRTSCTS;
}

We can double check once again with our designers to confirm that without FIFO
the only way for the hardware handshaking to work is to use a PDC.
So IMHO, your patch should not be applied, sorry!

Best regards,

Cyrille

Le 07/09/2016 à 18:13, Richard Genoud a écrit :
> Commit 1cf6e8fc8341 ("tty/serial: at91: fix RTS line management when
> hardware handshake is enabled") broke the hardware handshake when
> DMA is not used.
> 
> So, here's a summary:
> If DMA is NOT USED the mode should be ATMEL_US_USMODE_NORMAL because the
> controller can't drive the RTS pin (there's no way for it to know when
> we can't receive data anymore).
> 
> If DMA is USED along with FIFOs, the mode should be ATMEL_US_USMODE_HWHS
> because when there's a FIFO, it can know when it's full and then drive
> the RTS pin accordingly.
> NB: I didn't test this mode since my board (at91sam9g35-cm) has not the
> FIFO mechanism.
> 
> If DMA is USED WITHOUT FIFOs, then this is the bad case: the controller
> can't control the RTS pin, and we can't control it either by hand. So,
> the hardware handshake is disabled. (that's what commit 5be605ac9af9
> ("tty/serial: atmel: fix hardware handshake selection") did).
> 
> There's still something obscure to me:
> In the SAM9G35 datasheet, it's said that
> "Setting the USART to operate with hardware handshaking is performed by
> writing the USART_MODE field in US_MR to the value 0x2.
> [..] Using this mode requires using the DMA channel for reception."
> 
> So, according to this, it *should* be possible to use automatic hardware
> handshake with the DMA and without FIFO (sam9x5 have no FIFOs like
> samad controllers)
> 
> NB: -stable is not Cced because it doesn't cleanly apply on 4.1+
> 
> Signed-off-by: Richard Genoud <richard.genoud@xxxxxxxxx>
> Fixes: 1cf6e8fc8341 ("tty/serial: at91: fix RTS line management when hardware handshake is enabled")
> ---
>  drivers/tty/serial/atmel_serial.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
> index e9b4fbf88c2d..503fbc623227 100644
> --- a/drivers/tty/serial/atmel_serial.c
> +++ b/drivers/tty/serial/atmel_serial.c
> @@ -2130,14 +2130,26 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
>  	} else if ((termios->c_cflag & CRTSCTS) &&
>  		   !mctrl_gpio_use_rtscts(atmel_port->gpios)) {
>  		/*
> -		 * RS232 with hardware handshake (RTS/CTS)
> -		 * handled by the controller.
> +		 * Automatic hardware handshake (RTS/CTS) only work with
> +		 * DMA enabled and FIFOs.
> +		 * TODO: sam9x5 controllers don't have FIFOs like samad
> +		 * controllers, and yet, the datasheet says that the
> +		 * ATMEL_US_USMODE_HWHS can be used with (and only with) a DMA
> +		 * channel for reception.
>  		 */
> -		if (atmel_use_dma_rx(port) && !atmel_use_fifo(port)) {
> -			dev_info(port->dev, "not enabling hardware flow control because DMA is used");
> -			termios->c_cflag &= ~CRTSCTS;
> +		if (atmel_use_dma_rx(port)) {
> +			if (atmel_use_fifo(port)) {
> +				mode |= ATMEL_US_USMODE_HWHS;
> +			} else {
> +				dev_info(port->dev, "not enabling hardware flow control because DMA is used without fifo");
> +				termios->c_cflag &= ~CRTSCTS;
> +			}
>  		} else {
> -			mode |= ATMEL_US_USMODE_HWHS;
> +			/*
> +			 * if DMA is not used, tell the controller that it
> +			 * should not handle the RTS pin automatically
> +			 */
> +			mode |= ATMEL_US_USMODE_NORMAL;
>  		}
>  	} else {
>  		/* RS232 without hadware handshake or controlled by GPIOs */
>