Re: [PATCH v5 2/4] pwm: driver for qualcomm ipq6018 pwm block

From: Baruch Siach <baruch@tkos.co.il>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: linux-pwm@vger.kernel.org, devicetree@vger.kernel.org,
	linux-arm-msm@vger.kernel.org, Andy Gross <agross@kernel.org>,
	Rob Herring <robh+dt@kernel.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Kathiravan T <kathirav@codeaurora.org>,
	Thierry Reding <thierry.reding@gmail.com>,
	kernel@pengutronix.de, Robert Marko <robert.marko@sartura.hr>,
	Lee Jones <lee.jones@linaro.org>,
	linux-arm-kernel@lists.infradead.org,
	Balaji Prakash J <bjagadee@codeaurora.org>
Subject: Re: [PATCH v5 2/4] pwm: driver for qualcomm ipq6018 pwm block
Date: Thu, 22 Jul 2021 13:08:47 +0300	[thread overview]
Message-ID: <87k0lizmmo.fsf@tarshish> (raw)
In-Reply-To: <20210716070427.kv7w6imp2zoxhyz5@pengutronix.de>

Hi Uwe,

On Fri, Jul 16 2021, Uwe Kleine-König wrote:
> On Fri, Jul 16, 2021 at 08:51:20AM +0300, Baruch Siach wrote:
>> On Wed, Jul 14 2021, Uwe Kleine-König wrote:
>> > On Tue, Jul 13, 2021 at 02:35:43PM +0300, Baruch Siach wrote:
>> >> +	val = FIELD_PREP(IPQ_PWM_REG0_HI_DURATION, hi_dur) |
>> >> +		FIELD_PREP(IPQ_PWM_REG0_PWM_DIV, pwm_div);
>> >> +	ipq_pwm_reg_write(pwm, IPQ_PWM_CFG_REG0, val);
>> >> +
>> >> +	val = FIELD_PREP(IPQ_PWM_REG1_PRE_DIV, pre_div);
>> >> +	ipq_pwm_reg_write(pwm, IPQ_PWM_CFG_REG1, val);
>> >> +
>> >> +	/* Enable needs a separate write to REG1 */
>> >> +	val |= IPQ_PWM_REG1_UPDATE;
>> >
>> > Setting this bit results in the two writes above being configured
>> > atomically so that no mixed settings happen to the output, right?
>> 
>> I guess so. I have no access to hardware documentation, mind you. I
>> first tried to do only one write to REG1, but it had no effect. The
>> existence of the UPDATE bit also indicates that hardware works as you
>> suggest.
>
> I wouldn't trust HW documentation here. If you have some means to
> inspect the waveform this is easy to test. Depending on how long you can
> make the periods an LED is enough. If you start with a slower parent
> clk, a big pre_div and hi_dur = 0 the LED is supposed to be off. Then
> set hi_dur = pwm_div/2 which either make the LED blink slowly or keeps
> off. Then setting pre_div = 2 either increased the blink frequency or it
> doesn't. ...

I currently have only access to DVM to measure the PWM effect. I'll try
to do more measures when I have access to better equipment.

baruch

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