linux-kernel - Re: [PATCH] mfd: twl6040: Optional clk32k clock handling

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Date:	Mon, 28 Apr 2014 13:46:50 +0300
From:	Peter Ujfalusi <peter.ujfalusi@...com>
To:	Lee Jones <lee.jones@...aro.org>
CC:	<sameo@...ux.intel.com>, <devicetree@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mfd: twl6040: Optional clk32k clock handling

On 04/28/2014 01:36 PM, Lee Jones wrote:
>> In certain boards the source for the clk32k clock can be gated. In these
>> boards the clk32k clock can be provided to the driver and it is going to be
>> enabled/disabled when it is needed.
>> If the clk32k clock is not provided the driver will assume that it is always
>> running.
>>
>> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@...com>
>> ---
>>  Documentation/devicetree/bindings/mfd/twl6040.txt |  2 ++
>>  drivers/mfd/twl6040.c                             | 10 ++++++++++
>>  include/linux/mfd/twl6040.h                       |  2 ++
>>  3 files changed, 14 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/mfd/twl6040.txt b/Documentation/devicetree/bindings/mfd/twl6040.txt
>> index 0f5dd709d752..a41157b5d930 100644
>> --- a/Documentation/devicetree/bindings/mfd/twl6040.txt
>> +++ b/Documentation/devicetree/bindings/mfd/twl6040.txt
>> @@ -19,6 +19,8 @@ Required properties:
>>  
>>  Optional properties, nodes:
>>  - enable-active-high: To power on the twl6040 during boot.
>> +- clocks: phandle to the clk32k clock provider
>> +- clock-names: Must be "clk32k"
>>  
>>  Vibra functionality
>>  Required properties:
>> diff --git a/drivers/mfd/twl6040.c b/drivers/mfd/twl6040.c
>> index 2e6504a8e1e3..12b314ea48dc 100644
>> --- a/drivers/mfd/twl6040.c
>> +++ b/drivers/mfd/twl6040.c
>> @@ -291,6 +291,8 @@ int twl6040_power(struct twl6040 *twl6040, int on)
>>  		if (twl6040->power_count++)
>>  			goto out;
>>  
>> +		clk_prepare_enable(twl6040->clk32k);
>> +
>>  		/* Allow writes to the chip */
>>  		regcache_cache_only(twl6040->regmap, false);
>>  
>> @@ -346,6 +348,8 @@ int twl6040_power(struct twl6040 *twl6040, int on)
>>  
>>  		twl6040->sysclk = 0;
>>  		twl6040->mclk = 0;
>> +
>> +		clk_disable_unprepare(twl6040->clk32k);
>>  	}
>>  
>>  out:
>> @@ -644,6 +648,12 @@ static int twl6040_probe(struct i2c_client *client,
>>  
>>  	i2c_set_clientdata(client, twl6040);
>>  
>> +	twl6040->clk32k = devm_clk_get(&client->dev, "clk32k");
>> +	if (IS_ERR(twl6040->clk32k)) {
>> +		dev_info(&client->dev, "clk32k is not handled\n");
>> +		twl6040->clk32k = NULL;
> 
> So what happens if you pass a NULL clk reference to clk_prepare_enable()?

We have checks for !clk in all cases within the clk_* API. If we pass NULL,
nothing will happen and from the calling driver point of view it will look
like a success.
AFAIK this is the way to handle non esential/optional clocks without if()s in
the driver code around the clock handling.

-- 
Péter
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