linux-kernel - Re: [PATCH] Proper RTS control when buffers fill

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Message-ID: <20200107104950.GD30908@localhost>
Date:   Tue, 7 Jan 2020 11:49:50 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Brant Merryman <Brant.Merryman@...abs.com>
Cc:     Johan Hovold <johan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Richard Hendricks <Richard.Hendricks@...abs.com>
Subject: Re: [PATCH] Proper RTS control when buffers fill

On Wed, Dec 18, 2019 at 08:40:48PM +0000, Brant Merryman wrote:

You forgot to add the Subject prefix.

> 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: Brant Merryman <brant.merryman@...abs.com>
> ---
>  drivers/usb/serial/cp210x.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/drivers/usb/serial/cp210x.c b/drivers/usb/serial/cp210x.c
> index bcceb4ad8be0..74c9f3822bd2 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);
>  
> @@ -1015,8 +1016,24 @@ static void cp210x_get_termios_port(struct usb_serial_port *port,
>  	cp210x_read_reg_block(port, CP210X_GET_FLOW, &flow_ctl,
>  			sizeof(flow_ctl));
>  	ctl_hs = le32_to_cpu(flow_ctl.ulControlHandshake);
> +	flow_repl = le32_to_cpu(flow_ctl.ulFlowReplace);

Move inside the if block (after the comment).

>  	if (ctl_hs & CP210X_SERIAL_CTS_HANDSHAKE) {
>  		dev_dbg(dev, "%s - flow control = CRTSCTS\n", __func__);
> +		/*
> +		 * 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.
> +		 */

Maybe drop the "allows data to flow out" bit, the uart has been disabled
after all, right? And RTS is asserted on open by the TTY layer anyway so
data should flow in once opened.

As I asked you before, could you be more specific about what state
RTS-line end up in when disabling the UART (e.g. 0x00: statically
inactive)?

> +		flow_repl &= ~CP210X_SERIAL_RTS_MASK;
> +		flow_repl |= CP210X_SERIAL_RTS_SHIFT(CP210X_SERIAL_RTS_FLOW_CTL);
> +
> +		flow_ctl.ulControlHandshake = cpu_to_le32(ctl_hs);

Again, this line is not needed.

> +		flow_ctl.ulFlowReplace = cpu_to_le32(flow_repl);
> +		cp210x_write_reg_block(port, CP210X_SET_FLOW, &flow_ctl, sizeof(flow_ctl));

You still need to break this line. 

> +
>  		cflag |= CRTSCTS;
>  	} else {
>  		dev_dbg(dev, "%s - flow control = NONE\n", __func__);

Johan