linux-kernel - Re: [PATCH 1/4] serial: core: Add LED trigger support

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Message-ID: <20161124082626.ge7t64fwrtx3slvo@pengutronix.de>
Date:   Thu, 24 Nov 2016 09:26:26 +0100
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kernel@...gutronix.de,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 1/4] serial: core: Add LED trigger support

On Wed, Nov 23, 2016 at 11:08:19AM +0100, Greg Kroah-Hartman wrote:
> On Wed, Nov 23, 2016 at 11:01:03AM +0100, Sascha Hauer wrote:
> > With this patch the serial core provides LED triggers for RX and TX.
> > 
> > As the serial core layer does not know when the hardware actually sends
> > or receives characters, this needs help from the UART drivers. The
> > LED triggers are registered in uart_add_led_triggers() called from
> > the UART drivers which want to support LED triggers. All the driver
> > has to do then is to call uart_led_trigger_[tx|rx] to indicate
> > activity.

BTW last time LED triggers were discussed
(https://patchwork.kernel.org/patch/9212885/) You and Arnd mandated the
triggers should be implemented in the tty layer. By tty layer did you
really mean the tty layer or did you mean serial_core?

We could implement it in the tty layer, but tty doesn't know when the
characters are actually sent. There could be arbitrary time passing
between a tty_operations->put_char and the character being on the wire.

Also I am not sure if we want to have LED triggers for each and every
tty in the system

Sascha

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