Re: [PATCH v3 2/2] USB: serial: cp210x: Proper RTS control when buffers fill

[Johan Hovold <johan@kernel.org>]

On Fri, Jun 26, 2020 at 04:24:20AM +0000, Phu Luu wrote:
> From: Brant Merryman <brant.merryman@silabs.com>
> 
> CP210x hardware disables auto-RTS but leaves auto-CTS when
> in hardware flow control mode and UART on cp210x hardware
> is disabled. This allows data to flow out, but new data
> will not come into the port. When re-opening the port, if
> auto-CTS is enabled on the cp210x, then auto-RTS must be
> re-enabled in the driver.
> 
> Signed-off-by: Phu Luu <phu.luu@silabs.com>
> Signed-off-by: Brant Merryman <brant.merryman@silabs.com>

Please revisit these tags as well.

> ---
> 06/09/2020: Patch v3 2/2 Modified based on feedback from Johan Hovold <johan@kernel.org>
> 12/18/2019: Patch v2 Broken into two patches and modified based on feedback from Johan Hovold <johan@kernel.org>
> 12/09/2019: Initial submission of patch "Proper RTS control when buffers fill"
> 
>  drivers/usb/serial/cp210x.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/drivers/usb/serial/cp210x.c b/drivers/usb/serial/cp210x.c
> index bcceb4ad8be0..091441b1c5b9 100644
> --- a/drivers/usb/serial/cp210x.c
> +++ b/drivers/usb/serial/cp210x.c
> @@ -917,6 +917,7 @@ static void cp210x_get_termios_port(struct usb_serial_port *port,
>  	u32 baud;
>  	u16 bits;
>  	u32 ctl_hs;
> +	u32 flow_repl;
>  
>  	cp210x_read_u32_reg(port, CP210X_GET_BAUDRATE, &baud);
>  
> @@ -1017,6 +1018,23 @@ static void cp210x_get_termios_port(struct usb_serial_port *port,
>  	ctl_hs = le32_to_cpu(flow_ctl.ulControlHandshake);
>  	if (ctl_hs & CP210X_SERIAL_CTS_HANDSHAKE) {
>  		dev_dbg(dev, "%s - flow control = CRTSCTS\n", __func__);
> +		/*
> +		 * When the port is closed, the CP210x hardware disables
> +		 * auto-RTS and RTS is deasserted but it leaves auto-CTS when
> +		 * in hardware flow control mode. This prevents new data from
> +		 * being received by the port. When re-opening the port, if
> +		 * auto-CTS is enabled on the cp210x, then auto-RTS must be
> +		 * re-enabled in the driver.
> +		 */

I already asked this of Brant, but could you please be more specific
about which state the RTS-line end up in when disabling the UART (e.g.
0x00: statically inactive)?

The reason I ask is that after open() returns, the tty layer would raise
RTS, which should again allow data to flow in in contrast to what the
comment and changelog text claims.

We still want to enable auto-RTS of course so the patch is otherwise
correct.

> +		flow_repl = le32_to_cpu(flow_ctl.ulFlowReplace);
> +		flow_repl &= ~CP210X_SERIAL_RTS_MASK;
> +		flow_repl |= CP210X_SERIAL_RTS_SHIFT(CP210X_SERIAL_RTS_FLOW_CTL);
> +		flow_ctl.ulFlowReplace = cpu_to_le32(flow_repl);
> +		cp210x_write_reg_block(port,
> +				CP210X_SET_FLOW,
> +				&flow_ctl,
> +				sizeof(flow_ctl));
> +
>  		cflag |= CRTSCTS;
>  	} else {
>  		dev_dbg(dev, "%s - flow control = NONE\n", __func__);

Johan