linux-kernel - Re: [PATCH v2 2/2] tty: serial: msm: Disable pclk when port is closed

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Date:	Thu, 9 Apr 2015 11:06:14 +0530
From:	"Pramod Gurav" <gpramod@...eaurora.org>
To:	"Stephen Boyd" <sboyd@...eaurora.org>
Cc:	"Pramod Gurav" <gpramod@...eaurora.org>,
	linux-arm-msm@...r.kernel.org, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
	bryanh@...eaurora.org, jslaby@...e.cz
Subject: Re: [PATCH v2 2/2] tty: serial: msm: Disable pclk when port is
 closed


On Thu, April 9, 2015 5:21 am, Stephen Boyd wrote:
> On 04/08/15 06:28, Pramod Gurav wrote:
>> Disable the pclk when tty port is closed by user space.
>>
>> Signed-off-by: Pramod Gurav <gpramod@...eaurora.org>
>> ---
>>  drivers/tty/serial/msm_serial.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/tty/serial/msm_serial.c
>> b/drivers/tty/serial/msm_serial.c
>> index 4c1e9ea..f38565c 100644
>> --- a/drivers/tty/serial/msm_serial.c
>> +++ b/drivers/tty/serial/msm_serial.c
>> @@ -523,6 +523,7 @@ static void msm_shutdown(struct uart_port *port)
>>  	msm_write(port, 0, UART_IMR); /* disable interrupts */
>>
>>  	clk_disable_unprepare(msm_port->clk);
>> +	clk_disable_unprepare(msm_port->pclk);
>>
>>  	free_irq(port->irq, port);
>>  }
>
> It's not clear to me at all when this clock is enabled and when it's
> disabled during the lifetime of this driver. For example, why do we have
> a .pm op to turn clocks on and off? Shouldn't they already be on? Can
> you please explain when the clocks are turned on and off and what
> userspace actions cause that to happen? Looking at drivers like
> amba-pl010.c I don't see any .pm op, just a
> clk_prepare_enable/clk_disable_unprepare pair in the startup and
> shutdown ops.


When a userspce opens a serial port the uart_startup (in serial_core)
function is executed which changes the uart_pm state to UART_PM_STATE_ON.
So when this port is release/closed by the application
uart_close(serial_core) changes the uart_pm state to UART_PM_STATE_OFF if
its not console. But it is not done in uart_shutdown function.

So ideally clk_prepare_enable/clk_disable_unprepare must be called in .pm
only. So, we can get rid of these operations from msm_startup function as
these will be called anyway using .pm ops.

About .pm in uart_ops, there are few drivers which have it for an example,
atmel_serial, sh-sci etc. That is where they do
clk_prepare_enable/clk_disable_unprepare. And moreover when there is
suspend across system these function will handled through .pm.


>
> Minus my confusion of why our clocking is complicated, it looks correct
> to me to do this, so
>
> Reviewed-by: Stephen Boyd <sboyd@...eaurora.org>

>
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> a Linux Foundation Collaborative Project
>
>

Pramod

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project


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