Re: [RFC PATCH 06/10] pwm: ntxec: Add driver for PWM function in Netronix EC

On Mon, Jun 22, 2020 at 10:18:02AM +0200, Uwe Kleine-König wrote:

Hello,

On Sun, Jun 21, 2020 at 12:42:17AM +0200, Jonathan Neuschäfer wrote:

quoted

The Netronix EC provides a PWM output, which is used for the backlight

s/,//

quoted

on ebook readers. This patches adds a driver for the PWM output.

on *some* ebook readers

Ok, I'll fix these.

quoted

+#define NTXEC_UNK_A		0xa1
+#define NTXEC_UNK_B		0xa2
+#define NTXEC_ENABLE		0xa3
+#define NTXEC_PERIOD_LOW	0xa4
+#define NTXEC_PERIOD_HIGH	0xa5
+#define NTXEC_DUTY_LOW		0xa6
+#define NTXEC_DUTY_HIGH		0xa7
+
+/*
+ * The time base used in the EC is 8MHz, or 125ns. Period and duty cycle are
+ * measured in this unit.
+ */
+static int ntxec_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm_dev,
+				 int duty_ns, int period_ns)
+{
+	struct ntxec_pwm *pwm = pwmchip_to_pwm(chip);
+	uint64_t duty = duty_ns;
+	uint64_t period = period_ns;

As you cannot use values bigger than 8191999 anyhow, I wonder why you
use a 64 bit type here.

No particular reason; I possibly got confused by the division API. I'll
use uint32_t instead.

quoted

+	int res = 0;
+
+	do_div(period, 125);

Please use a define instead of plain 125.

Will do.

quoted

+	if (period > 0xffff) {
+		dev_warn(pwm->dev,
+			 "Period is not representable in 16 bits: %llu\n", period);
+		return -ERANGE;
+	}
+
+	do_div(duty, 125);
+	if (duty > 0xffff) {
+		dev_warn(pwm->dev, "Duty cycle is not representable in 16 bits: %llu\n",
+			duty);
+		return -ERANGE;
+	}

This check isn't necessary as the pwm core ensures that duty <= period.

Ok, I'll remove it.

quoted

+	res |= ntxec_write8(pwm->ec, NTXEC_PERIOD_HIGH, period >> 8);
+	res |= ntxec_write8(pwm->ec, NTXEC_PERIOD_LOW, period);
+	res |= ntxec_write8(pwm->ec, NTXEC_DUTY_HIGH, duty >> 8);
+	res |= ntxec_write8(pwm->ec, NTXEC_DUTY_LOW, duty);

Does this complete the currently running period? Can it happen that a
new period starts between the first and the last write and so a mixed
period can be seen at the output?

Good question. I haven't measured it, and also don't have the code
running on the EC.

quoted

+
+	return (res < 0) ? -EIO : 0;
+}
+
+static int ntxec_pwm_enable(struct pwm_chip *chip,
+				 struct pwm_device *pwm_dev)
+{
+	struct ntxec_pwm *pwm = pwmchip_to_pwm(chip);
+
+	return ntxec_write8(pwm->ec, NTXEC_ENABLE, 1);
+}
+
+static void ntxec_pwm_disable(struct pwm_chip *chip,
+				   struct pwm_device *pwm_dev)
+{
+	struct ntxec_pwm *pwm = pwmchip_to_pwm(chip);
+
+	ntxec_write8(pwm->ec, NTXEC_ENABLE, 0);
+}
+
+static struct pwm_ops ntxec_pwm_ops = {
+	.config		= ntxec_pwm_config,
+	.enable		= ntxec_pwm_enable,
+	.disable	= ntxec_pwm_disable,
+	.owner		= THIS_MODULE,

Please don't align the =, just a single space before them is fine.

Ok

More important: Please implement .apply() (and .get_state()) instead of
the old API. Also please enable PWM_DEBUG which might save us a review
iteration.

Will do!

quoted

+};
+
+static int ntxec_pwm_probe(struct platform_device *pdev)
+{
+	struct ntxec *ec = dev_get_drvdata(pdev->dev.parent);
+	struct ntxec_pwm *pwm;
+	struct pwm_chip *chip;
+	int res;
+
+	pwm = devm_kzalloc(&pdev->dev, sizeof(*pwm), GFP_KERNEL);
+	if (!pwm)
+		return -ENOMEM;
+
+	pwm->ec = ec;
+	pwm->dev = &pdev->dev;
+
+	chip = &pwm->chip;
+	chip->dev = &pdev->dev;
+	chip->ops = &ntxec_pwm_ops;
+	chip->base = -1;
+	chip->npwm = 1;
+
+	res = pwmchip_add(chip);
+	if (res < 0)
+		return res;
+
+	platform_set_drvdata(pdev, pwm);
+
+	res |= ntxec_write8(pwm->ec, NTXEC_ENABLE, 0);
+	res |= ntxec_write8(pwm->ec, NTXEC_UNK_A, 0xff);
+	res |= ntxec_write8(pwm->ec, NTXEC_UNK_B, 0xff);
+
+	return (res < 0) ? -EIO : 0;

This is broken for several reasons:

 - You're not supposed to modify the output in .probe
 - if ntxec_write8 results in an error you keep the pwm registered.
 - From the moment on pwmchip_add returns the callbacks can be called.
   The calls to ntxec_write8 probably interfere here.

Ok, I'll rework the probe function to avoid these issues.


Thanks for the review,
Jonathan Neuschäfer