linux-kernel - Re: [PATCH v8 1/2] PWM: atmel-pwm: add PWM controller driver

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Date:	Thu, 5 Dec 2013 09:11:28 +0800
From:	Bo Shen <voice.shen@...el.com>
To:	Thierry Reding <thierry.reding@...il.com>
CC:	Bo Shen <voice.shen@...el.com>, <linux-pwm@...r.kernel.org>,
	<devicetree@...r.kernel.org>, <nicolas.ferre@...el.com>,
	<linux-kernel@...r.kernel.org>,
	<alexandre.belloni@...e-electrons.com>, <galak@...eaurora.org>,
	<plagnioj@...osoft.com>, <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v8 1/2] PWM: atmel-pwm: add PWM controller driver

Hi Thierry,

On 12/04/2013 06:03 PM, Thierry Reding wrote:
> On Wed, Dec 04, 2013 at 10:59:46AM +0800, Bo Shen wrote:
>> Hi Thierry,
>>
>> On 12/03/2013 05:43 PM, Thierry Reding wrote:
>>> On Tue, Dec 03, 2013 at 11:09:12AM +0800, Bo Shen wrote:
>>>> On 12/02/2013 06:59 PM, Thierry Reding wrote:
>>>>> On Mon, Nov 18, 2013 at 05:13:21PM +0800, Bo Shen wrote:
>>> [...]
>>>>>> diff --git a/drivers/pwm/pwm-atmel.c b/drivers/pwm/pwm-atmel.c
>>> [...]
>>>>>> +	/* Calculate the period cycles */
>>>>>> +	while (div > PWM_MAX_PRD) {
>>>>>> +		div = clk_rate / (1 << pres);
>>>>>> +		div = div * period_ns;
>>>>>> +		/* 1/Hz = 100000000 ns */
>>>>>
>>>>> I don't think that comment is needed.
>>>>
>>>> This is asked to be added.
>>>> And, I think keep it and it won't hurt, what do you think?
>>>
>>> I think it's obvious that you're converting from nanoseconds because of
>>> the _ns prefix in period_ns. But if somebody requested this and everyone
>>> else thinks it's useful, I'm okay with keeping it.
>>>
>>>>>> +	if (test_bit(PWMF_ENABLED, &pwm->flags)) {
>>>>>> +		atmel_pwm_ch_writel(atmel_pwm, pwm->hwpwm, PWMV2_CDTYUPD, dty);
>>>>>> +	} else {
>>>>>> +		atmel_pwm_ch_writel(atmel_pwm, pwm->hwpwm, PWMV2_CDTY, dty);
>>>>>> +		atmel_pwm_ch_writel(atmel_pwm, pwm->hwpwm, PWMV2_CPRD, prd);
>>>>>> +	}
>>>>>> +}
>>>>>
>>>>> Neither version 1 nor version 2 seem to be able to change the period
>>>>> while the channel is enabled. Perhaps that should be checked for in
>>>>> atmel_pwm_config() and an error (-EBUSY) returned?
>>>>
>>>> The period is configured in dt in device tree, or platform data in non
>>>> device tree. Nowhere will update period. So, not code to update period.
>>>> Am I right? If not, please figure out.
>>>
>>> The .config() operation allows the period to be specified. Just because
>>> nobody currently changes it at runtime doesn't mean it can't be done.
>>>
>>> It is also possible that whoever wrote the device tree or platform data
>>> didn't know that the period must be the same for all PWM channels and
>>> therefore might use different values. If you check for this, at least
>>> they'll notice. If you don't check for it, then they may end up having
>>> the period reconfigured behind their backs, which may cause parts of
>>> their setup to behave unexpectedly.
>>
>> Thanks for this information.
>> I will add code for changing period.
>
> Just to clarify: I wouldn't want this code to allow changing the period
> but rather reject incompatible changes to the period with an error code.

So, in this patch, just check it as you suggested in previous email, 
would it be OK?
--->8---
Perhaps that should be checked for in atmel_pwm_config() and an error 
(-EBUSY) returned?
---8<---

> Thierry

Best Regards,
Bo Shen

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