linux-kernel - Re: [PATCH v2] USB: serial: CH341: add hardware flow control RTS/CTS

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Message-ID: <Z35P1UNKlGzNSQiU@hovoldconsulting.com>
Date: Wed, 8 Jan 2025 11:13:41 +0100
From: Johan Hovold <johan@...nel.org>
To: Lode Willems <me@...ewillems.com>
Cc: gregkh@...uxfoundation.org, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] USB: serial: CH341: add hardware flow control RTS/CTS

On Wed, Dec 18, 2024 at 06:25:05PM +0100, Lode Willems wrote:
> This adds support for enabling and disabling
> RTS/CTS hardware flow control.
> Tested using CH341A and CH340E.
> 
> Fixes part of the following bug report:
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=197109
> 
> Signed-off-by: Lode Willems <me@...ewillems.com>
> ---
> 
> Changes in v2:
>  - Change the CH341_FLOW_CTL_* values to be 8 bits wide
>  - Set flow control setting unconditionally
>  - Prepare parameters and do one control request
>  - Drop the error message

Thanks for the update.

> +#define CH341_REG_FLOW_CTL     0x27

> +#define CH341_FLOW_CTL_NONE    0x00
> +#define CH341_FLOW_CTL_RTSCTS  0x01

> +static void ch341_set_flow_control(struct tty_struct *tty,
> +				   struct usb_serial_port *port,
> +				   const struct ktermios *old_termios)
> +{
> +	int r;
> +	u16 flow_ctl;
> +
> +	if (C_CRTSCTS(tty))
> +		flow_ctl = ((u16)CH341_FLOW_CTL_RTSCTS << 8) | CH341_FLOW_CTL_RTSCTS;
> +	else
> +		flow_ctl = ((u16)CH341_FLOW_CTL_NONE << 8) | CH341_FLOW_CTL_NONE;
> +
> +	r = ch341_control_out(port->serial->dev,
> +			      CH341_REQ_WRITE_REG,
> +			      ((u16)CH341_REG_FLOW_CTL << 8) | CH341_REG_FLOW_CTL,
> +			      flow_ctl);

I dropped the unnecessary casts above (and will follow-up with a patch
removing one such cast from the existing code). I also think preparing
the index argument once in the call (i.e. (flow_ctl << 8) | flow_ctl))
makes the code a bit more readable.

End result is here:

	https://git.kernel.org/pub/scm/linux/kernel/git/johan/usb-serial.git/commit/?h=usb-next&id=35478bc369a67b703a079ee123c6e58290114aae	

Johan