linux-kernel - Re: [PATCH 2/3] USB : serial : invoke dcd

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Date:	Fri, 13 Sep 2013 17:53:55 +0200
From:	Johan Hovold <jhovold@...il.com>
To:	Paul Chavent <Paul.Chavent@...ra.fr>
Cc:	linux-usb@...r.kernel.org, gregkh@...uxfoundation.org,
	jhovold@...il.com, fschaefer.oss@...glemail.com, jslaby@...e.cz,
	max@...e.de, giometti@...eenne.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] USB : serial : invoke dcd_change ldisc's handler.

On Fri, Sep 13, 2013 at 05:35:12PM +0200, Paul Chavent wrote:
> Signed-off-by: Paul Chavent <paul.chavent@...ra.fr>
> ---
>  Documentation/pps/pps.txt    | 15 +++++++++++++++
>  drivers/usb/serial/generic.c |  9 +++++++++
>  2 files changed, 24 insertions(+)
> 
> diff --git a/Documentation/pps/pps.txt b/Documentation/pps/pps.txt
> index d35dcdd..67b9a94 100644
> --- a/Documentation/pps/pps.txt
> +++ b/Documentation/pps/pps.txt
> @@ -66,6 +66,21 @@ In LinuxPPS the PPS sources are simply char devices usually mapped
>  into files /dev/pps0, /dev/pps1, etc..
>  
>  
> +PPS with USB to serial devices
> +------------------------------
> +
> +It is possible to grab the PPS from an USB to serial device. However,
> +you should take into account the latencies and jitter introduced by
> +the USB stack. Users has reported clock instability around +-1ms when
> +synchronized with PPS through USB. This isn't suited for time server
> +synchronisation.
> +
> +If your device doesn't report PPS, you can check that the feature is
> +supported by its driver. Most of the time, you only need to add a call
> +to usb_serial_handle_dcd_change after checking the DCD status (see
> +ch341 and pl2303 examples).
> +
> +
>  Coding example
>  --------------
>  
> diff --git a/drivers/usb/serial/generic.c b/drivers/usb/serial/generic.c
> index 1f31e6b..877d6e0 100644
> --- a/drivers/usb/serial/generic.c
> +++ b/drivers/usb/serial/generic.c
> @@ -568,6 +568,15 @@ void usb_serial_handle_dcd_change(struct usb_serial_port *usb_port,
>  {
>  	struct tty_port *port = &usb_port->port;
>  
> +	if (tty) {
> +		struct tty_ldisc *ld = tty_ldisc_ref(tty);
> +		if (ld) {
> +			if (ld->ops->dcd_change)
> +				ld->ops->dcd_change(tty, status);
> +			tty_ldisc_deref(ld);
> +		}
> +	}
> +
>  	dev_dbg(&usb_port->dev, "%s - status %d\n", __func__, status);

Please add your ldisc handling after the debug statement.

Thanks,
Johan
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