linux-kernel - Re: [PATCH v2 2/4] pwm: kona: Fix incorrect enable after channel polarity change

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Message-ID: <547909B0.2090408@broadcom.com>
Date:	Fri, 28 Nov 2014 15:48:00 -0800
From:	Arun Ramamurthy <arun.ramamurthy@...adcom.com>
To:	Tim Kryger <tim.kryger@...il.com>,
	Scott Branden <sbranden@...adcom.com>
CC:	Thierry Reding <thierry.reding@...il.com>,
	Ray Jui <rjui@...adcom.com>,
	Arun Ramamurthy <arunrama@...adcom.com>,
	<bcm-kernel-feedback-list@...adcom.com>,
	<linux-pwm@...r.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/4] pwm: kona: Fix incorrect enable after channel
 polarity change



On 14-11-25 10:22 PM, Tim Kryger wrote:
> On Tue, Nov 25, 2014 at 11:40 AM, Scott Branden <sbranden@...adcom.com> wrote:
>> From: Arun Ramamurthy <arunrama@...adcom.com>
>>
>> The pwm core code requires a separate call for enabling the channel
>> and hence the driver does not need to set pwm_trigger after a
>> polarity change
>
> The framework does restrict when polarity changes can occur but it
> isn't clear to me that there is any reason to delay applying the
> polarity change.
I examined several other drivers such as pwm-atmel-tcb.c, pwm-ep93xx.c, 
pwm-renesas-tpu.c, pwm-samsung.c in the 3.17 kernel tree and none of 
them enable the channel after changing polarity. We would be the first 
driver to do so.

> Keep in mind that polarity matters even when a PWM
> is disabled.  While disabled, the output should be equivalent to an
> enabled configuration with zero duty.  Thus for normal polarity the
> output is constant low and for inversed polarity the output is
> constant high.
The driver does set the duty cycle to zero when disabling the pwm 
channel.However since the frame work prevents polarity change when the 
pwm is enabled, I don’t see how one could expect the polarity change to 
be reflected immediately without a separate call to pwm enable.


I believe there is an expectation that the output is
> updated to reflect the requested polarity change prior to returning to
> the caller.

Once again I disagree with this based on other pwm drivers which only 
change the polarity and do not enable the channel when their set 
polarity functions are called.

>
>>
>> Signed-off-by: Arun Ramamurthy <arunrama@...adcom.com>
>> Reviewed-by: Ray Jui <rjui@...adcom.com>
>> Signed-off-by: Scott Branden <sbranden@...adcom.com>
>> ---
>>   drivers/pwm/pwm-bcm-kona.c | 5 -----
>>   1 file changed, 5 deletions(-)
>>
>> diff --git a/drivers/pwm/pwm-bcm-kona.c b/drivers/pwm/pwm-bcm-kona.c
>> index 29eef9e..fa0b5bf 100644
>> --- a/drivers/pwm/pwm-bcm-kona.c
>> +++ b/drivers/pwm/pwm-bcm-kona.c
>> @@ -173,11 +173,6 @@ static int kona_pwmc_set_polarity(struct pwm_chip *chip, struct pwm_device *pwm,
>>
>>          writel(value, kp->base + PWM_CONTROL_OFFSET);
>>
>> -       kona_pwmc_apply_settings(kp, chan);
>> -
>> -       /* Wait for waveform to settle before gating off the clock */
>> -       ndelay(400);
>> -
>>          clk_disable_unprepare(kp->clk);
>>
>>          return 0;
>> --
>> 2.1.3
>>
--
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