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

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Message-ID: <e9fc12b8-d4a7-067d-ef87-03f0aae39bd7@gmail.com>
Date:   Wed, 23 Nov 2016 10:57:13 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Sascha Hauer <s.hauer@...gutronix.de>, linux-serial@...r.kernel.org
Cc:     kernel@...gutronix.de,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/4] serial: core: Add LED trigger support

On 11/23/2016 02:01 AM, 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.

Looking at 8250, we call serial8250_tx_chars from __start_tx which is
called form the uart_ops::start_tx, for RX I sort of agree, since this
happens in interrupt handler. I suppose some drivers could actually
queue work but not do it right away though?

> 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.

Could we somehow remedy the lack of knowledge from the core as whether
the HW sends/receives characters first before adding support for LED
triggers? It would be more generic and future proof to require UART
drivers to report to the core when they actually TX/RX, and then at the
core level, utilize that knowledge to perform the LED trigger.

Side note: are you positive using drv->dev_name is robust enough on
systems with many different UART drivers, yet all of them being ttyS*?

Thanks!
-- 
Florian