Re: [PATCH 1/3] pwm: driver for qualcomm ipq6018 pwm block

From: Baruch Siach <baruch@tkos.co.il>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: Thierry Reding <thierry.reding@gmail.com>,
	Lee Jones <lee.jones@linaro.org>, Andy Gross <agross@kernel.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Balaji Prakash J <bjagadee@codeaurora.org>,
	Rob Herring <robh+dt@kernel.org>,
	Robert Marko <robert.marko@sartura.hr>,
	devicetree@vger.kernel.org, linux-arm-msm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-pwm@vger.kernel.org
Subject: Re: [PATCH 1/3] pwm: driver for qualcomm ipq6018 pwm block
Date: Sun, 23 May 2021 18:54:08 +0300	[thread overview]
Message-ID: <87zgwltpi7.fsf@tarshish> (raw)
In-Reply-To: <20210522213524.lnb5bds5hvv2f2zi@pengutronix.de>

Hi Uwe,

Thanks for your review comments.

On Sun, May 23 2021, Uwe Kleine-König wrote:
> On Wed, May 19, 2021 at 10:48:44AM +0300, Baruch Siach wrote:
>> Driver for the PWM block in Qualcomm IPQ6018 line of SoCs. Based on
>> driver from downstream Codeaurora kernel tree. Removed support for older
>> (V1) variants because I have no access to that hardware.
>> 
>> Tested on IPQ6010 based hardware.
>> 
>> Signed-off-by: Baruch Siach <baruch@tkos.co.il>

[...]

>> +static void ipq_pwm_disable(struct pwm_chip *chip, struct pwm_device *pwm)
>> +{
>> +	struct ipq_pwm_chip *ipq_chip = to_ipq_pwm_chip(pwm->chip);
>> +	unsigned offset = ipq_pwm_reg_offset(pwm, PWM_CFG_REG1);
>> +	unsigned long val;
>> +
>> +	val = readl(ipq_chip->mem + offset);
>> +	val |= PWM_UPDATE;
>
> What is the effect of this register bit?
>
> Does the output become inactive or does it freeze at state that happens
> to be emitted when the ENABLE bit is removed?

I don't know. PWM does not work when this bit is not set here. The
original downstream driver[1] does not set this bit on disable. But it
also enables PWM unconditionally on .config. I added the 'enabled' check
in .config, and then PWM stopped working even when enabled later. It was
only by accident (excess copy/paste) that I found this workaround.

A comment on the original code says that PWM_UPDATE is "auto cleared".
This is evidently not true on my hardware (IPQ6010). This might be true
for older variants of this PWM block. Unfortunately, I have no access to
hardware documentation.

[1] https://source.codeaurora.org/quic/qsdk/oss/kernel/linux-ipq-5.4/tree/drivers/pwm/pwm-ipq.c?h=NHSS.QSDK.11.4.1.r1&id=9e4627b7088b0c06ddd910c8770274d26613de9e

baruch

-- 
                                                     ~. .~   Tk Open Systems
=}------------------------------------------------ooO--U--Ooo------------{=
   - baruch@tkos.co.il - tel: +972.52.368.4656, http://www.tkos.co.il -