Subject: Re: [PATCH v3 3/3] pwm: core: add consumer device link

From: Thierry Reding <[email-masked]>
Date: Wed, 13 Feb 2019 13:53:53 +0100

On Wed, Feb 13, 2019 at 11:50:12AM +0100, Fabrice Gasnier wrote:
> Add a device link between the PWM consumer and the PWM provider. This
> enforces the PWM user to get suspended before the PWM provider. It
> allows proper synchronization of suspend/resume sequences: the PWM user
> is responsible for properly stopping PWM, before the provider gets
> suspended: see [1]. Add the device link in:
> - of_pwm_get()
> - pwm_get()
> - devm_ variants
> as it requires a reference to the device for the PWM consumer.
> 
> [1] https://lkml.org/lkml/2019/2/5/770
> 
> Suggested-by: Thierry Reding <[email-masked]>
> Signed-off-by: Fabrice Gasnier <[email-masked]>
> ---
> Changes in v3:
> - add struct device to of_get_pwm() arguments to handle device link from
>   there.
> ---
>  drivers/pwm/core.c  | 14 +++++++++++---
>  include/linux/pwm.h |  6 ++++--
>  2 files changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> index 1581f6a..8cb5d4bc 100644
> --- a/drivers/pwm/core.c
> +++ b/drivers/pwm/core.c
> @@ -638,6 +638,7 @@ static struct pwm_chip *of_node_to_pwmchip(struct device_node *np)
>  
>  /**
>   * of_pwm_get() - request a PWM via the PWM framework
> + * @dev: device for PWM consumer
>   * @np: device node to get the PWM from
>   * @con_id: consumer name
>   *
> @@ -655,7 +656,8 @@ static struct pwm_chip *of_node_to_pwmchip(struct device_node *np)
>   * Returns: A pointer to the requested PWM device or an ERR_PTR()-encoded
>   * error code on failure.
>   */
> -struct pwm_device *of_pwm_get(struct device_node *np, const char *con_id)
> +struct pwm_device *of_pwm_get(struct device *dev, struct device_node *np,
> +			      const char *con_id)
>  {
>  	struct pwm_device *pwm = NULL;
>  	struct of_phandle_args args;
> @@ -689,6 +691,9 @@ struct pwm_device *of_pwm_get(struct device_node *np, const char *con_id)
>  	if (IS_ERR(pwm))
>  		goto put;
>  
> +	if (!device_link_add(dev, pwm->chip->dev, DL_FLAG_AUTOREMOVE_CONSUMER))
> +		pr_debug("%s(): device link not added\n", __func__);

I think it's better to turn this into dev_dbg(dev, ...) and maybe
mention which supplier it failed to link to, something like:

	if (!device_link_add(...))
		dev_dbg(dev, "failed to create device link to %s\n",
			pwm->chip->dev);

Also, I wonder if this should perhaps be dev_err(). Under what
circumstances does this fail?

> +
>  	/*
>  	 * If a consumer name was not given, try to look it up from the
>  	 * "pwm-names" property if it exists. Otherwise use the name of
> @@ -771,7 +776,7 @@ struct pwm_device *pwm_get(struct device *dev, const char *con_id)
>  
>  	/* look up via DT first */
>  	if (IS_ENABLED(CONFIG_OF) && dev && dev->of_node)
> -		return of_pwm_get(dev->of_node, con_id);
> +		return of_pwm_get(dev, dev->of_node, con_id);
>  
>  	/*
>  	 * We look up the provider in the static table typically provided by
> @@ -851,6 +856,9 @@ struct pwm_device *pwm_get(struct device *dev, const char *con_id)
>  	pwm->args.period = chosen->period;
>  	pwm->args.polarity = chosen->polarity;
>  
> +	if (!device_link_add(dev, pwm->chip->dev, DL_FLAG_AUTOREMOVE_CONSUMER))
> +		pr_debug("%s(): device link not added\n", __func__);

Same here. Also: not sure if we really need to include __func__ in the
message.

> +
>  	return pwm;
>  }
>  EXPORT_SYMBOL_GPL(pwm_get);
> @@ -939,7 +947,7 @@ struct pwm_device *devm_of_pwm_get(struct device *dev, struct device_node *np,
>  	if (!ptr)
>  		return ERR_PTR(-ENOMEM);
>  
> -	pwm = of_pwm_get(np, con_id);
> +	pwm = of_pwm_get(dev, np, con_id);
>  	if (!IS_ERR(pwm)) {
>  		*ptr = pwm;
>  		devres_add(dev, ptr);
> diff --git a/include/linux/pwm.h b/include/linux/pwm.h
> index d5199b5..895e074 100644
> --- a/include/linux/pwm.h
> +++ b/include/linux/pwm.h
> @@ -406,7 +406,8 @@ struct pwm_device *of_pwm_xlate_with_flags(struct pwm_chip *pc,
>  		const struct of_phandle_args *args);
>  
>  struct pwm_device *pwm_get(struct device *dev, const char *con_id);
> -struct pwm_device *of_pwm_get(struct device_node *np, const char *con_id);
> +struct pwm_device *of_pwm_get(struct device *dev, struct device_node *np,
> +			      const char *con_id);

I'm slightly concerned about this. I think one of the reasons why this
was introduced was to allow code to get at the PWM if they didn't have
a struct device * available. However, it doesn't seem like there are any
users of that function, so this seems fine.

Thierry

>  void pwm_put(struct pwm_device *pwm);
>  
>  struct pwm_device *devm_pwm_get(struct device *dev, const char *con_id);
> @@ -494,7 +495,8 @@ static inline struct pwm_device *pwm_get(struct device *dev,
>  	return ERR_PTR(-ENODEV);
>  }
>  
> -static inline struct pwm_device *of_pwm_get(struct device_node *np,
> +static inline struct pwm_device *of_pwm_get(struct device *dev,
> +					    struct device_node *np,
>  					    const char *con_id)
>  {
>  	return ERR_PTR(-ENODEV);
> -- 
> 1.9.1
> 
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