linux-kernel - Re: [PATCH v7 11/11] pwm: Add Raspberry Pi Firmware based PWM bus

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Message-ID: <20210310115041.s7tzvgdpksws6yss@pengutronix.de>
Date:   Wed, 10 Mar 2021 12:50:41 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     f.fainelli@...il.com, linux-kernel@...r.kernel.org,
        linux-pwm@...r.kernel.org, bcm-kernel-feedback-list@...adcom.com,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        wahrenst@....net, linux-input@...r.kernel.org,
        dmitry.torokhov@...il.com, gregkh@...uxfoundation.org,
        devel@...verdev.osuosl.org, p.zabel@...gutronix.de,
        linux-gpio@...r.kernel.org, linus.walleij@...aro.org,
        linux-clk@...r.kernel.org, sboyd@...nel.org,
        linux-rpi-kernel@...ts.infradead.org, bgolaszewski@...libre.com,
        andy.shevchenko@...il.com
Subject: Re: [PATCH v7 11/11] pwm: Add Raspberry Pi Firmware based PWM bus

Hello Nicolas,

On Mon, Jan 18, 2021 at 01:32:44PM +0100, Nicolas Saenz Julienne wrote:
> diff --git a/drivers/pwm/pwm-raspberrypi-poe.c b/drivers/pwm/pwm-raspberrypi-poe.c
> new file mode 100644
> index 000000000000..ca845e8fabe6
> --- /dev/null
> +++ b/drivers/pwm/pwm-raspberrypi-poe.c
> @@ -0,0 +1,220 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright 2020 Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
> + * For more information on Raspberry Pi's PoE hat see:
> + * https://www.raspberrypi.org/products/poe-hat/
> + *
> + * Limitations:
> + *  - No disable bit, so a disabled PWM is simulated by duty_cycle 0
> + *  - Only normal polarity
> + *  - Fixed 12.5 kHz period
> + *
> + * The current period is completed when HW is reconfigured.

nice.

> + */
> +
> [...]
> +static int raspberrypi_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> +				 const struct pwm_state *state)
> +{
> +	struct raspberrypi_pwm *rpipwm = raspberrypi_pwm_from_chip(chip);
> +	unsigned int duty_cycle;
> +	int ret;
> +
> +	if (state->period < RPI_PWM_PERIOD_NS ||
> +	    state->polarity != PWM_POLARITY_NORMAL)
> +		return -EINVAL;
> +
> +	if (!state->enabled)
> +		duty_cycle = 0;
> +	else if (state->duty_cycle < RPI_PWM_PERIOD_NS)
> +		duty_cycle = DIV_ROUND_DOWN_ULL(state->duty_cycle * RPI_PWM_MAX_DUTY,
> +						RPI_PWM_PERIOD_NS);
> +	else
> +		duty_cycle = RPI_PWM_MAX_DUTY;
> +
> +	if (duty_cycle == rpipwm->duty_cycle)
> +		return 0;
> +
> +	ret = raspberrypi_pwm_set_property(rpipwm->firmware, RPI_PWM_CUR_DUTY_REG,
> +					   duty_cycle);
> +	if (ret) {
> +		dev_err(chip->dev, "Failed to set duty cycle: %pe\n",
> +			ERR_PTR(ret));
> +		return ret;
> +	}
> +
> +	/*
> +	 * This sets the default duty cycle after resetting the board, we
> +	 * updated it every time to mimic Raspberry Pi's downstream's driver
> +	 * behaviour.
> +	 */
> +	ret = raspberrypi_pwm_set_property(rpipwm->firmware, RPI_PWM_DEF_DUTY_REG,
> +					   duty_cycle);
> +	if (ret) {
> +		dev_err(chip->dev, "Failed to set default duty cycle: %pe\n",
> +			ERR_PTR(ret));
> +		return ret;

This only has an effect for the next reboot, right? If so I wonder if it
is a good idea in general. (Think: The current PWM setting enables a
motor that makes a self-driving car move at 100 km/h. Consider the rpi
crashes, do I want to car to pick up driving 100 km/h at power up even
before Linux is up again?) And if we agree it's a good idea: Should
raspberrypi_pwm_apply return 0 if setting the duty cycle succeeded and
only setting the default didn't?

Other than that the patch looks fine.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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