linux-kernel - Re: [PATCH 4/5] tty: serial: 8250 core: add runtime pm

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Message-ID: <53C6A050.2050409@linutronix.de>
Date:	Wed, 16 Jul 2014 17:54:56 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	balbi@...com
CC:	linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Tony Lindgren <tony@...mide.com>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	mika.westerberg@...ux.intel.com
Subject: Re: [PATCH 4/5] tty: serial: 8250 core: add runtime pm

On 07/16/2014 05:16 PM, Felipe Balbi wrote:
> Hi,

Hi Felipe,

> On Wed, Jul 16, 2014 at 04:45:02PM +0200, Sebastian Andrzej Siewior
> wrote:
>> @@ -1280,6 +1285,7 @@ static void serial8250_stop_tx(struct
>> uart_port *port) struct uart_8250_port *up = container_of(port,
>> struct uart_8250_port, port);
>> 
>> +	pm_runtime_get_sync(port->dev); __stop_tx(up);
>> 
>> /* @@ -1289,6 +1295,8 @@ static void serial8250_stop_tx(struct
>> uart_port *port) up->acr |= UART_ACR_TXDIS; serial_icr_write(up,
>> UART_ACR, up->acr); } +	pm_runtime_mark_last_busy(port->dev); +
>> pm_runtime_put_autosuspend(port->dev); }
>> 
>> static void serial8250_start_tx(struct uart_port *port) @@
>> -1296,8 +1304,9 @@ static void serial8250_start_tx(struct
>> uart_port *port) struct uart_8250_port *up = container_of(port,
>> struct uart_8250_port, port);
>> 
>> +	pm_runtime_get_sync(port->dev); if (up->dma &&
>> !serial8250_tx_dma(up)) { -		return; +		goto out; } else if
>> (!(up->ier & UART_IER_THRI)) { up->ier |= UART_IER_THRI; 
>> serial_port_out(port, UART_IER, up->ier); @@ -1318,6 +1327,9 @@
>> static void serial8250_start_tx(struct uart_port *port) up->acr
>> &= ~UART_ACR_TXDIS; serial_icr_write(up, UART_ACR, up->acr); } 
>> +out: +	pm_runtime_mark_last_busy(port->dev); +
>> pm_runtime_put_autosuspend(port->dev);
> 
> I wonder if you should get_sync() on start_tx() and only 
> put_autosuspend() at stop_tx(). I guess the outcome would be
> largely the same, no ?

I just opened minicom on ttyS0 and gave a try. start_tx() was invoked
each time I pressed a key (sent a character). I haven't seen stop_tx()
even after after I closed minicom. I guess stop_tx() is invoked if you
switch half-duplex communication.

> well, other than in probe and other functions which need to make
> sure clocks are on, but it seems unnecessary to enable/disable in
> every function.

What do you have in mind? Do you plan to let the uart on while the
minicom is attached but is doing nothing? In that case, ->startup() and
->shutdown() should be enough.
If you want to put the uart off while the port is open but idle then
you should cover the callbacks as they are independent of each other.
You could receive three ->set_termios() even without a single
->start_tx(). And as far as I understand the whole situation, you need
to make sure the UART is "up" while you touch a single register not
only sending characters, right?

Sebastian
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