linux-kernel - Re: [RFC PATCH v3] media: v4l2-common: Add a helper for obtaining the clock producer

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Message-ID: <20250321101124.GB25483@pendragon.ideasonboard.com>
Date: Fri, 21 Mar 2025 12:11:24 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Mehdi Djait <mehdi.djait@...ux.intel.com>
Cc: sakari.ailus@...ux.intel.com, tomi.valkeinen@...asonboard.com,
	jacopo.mondi@...asonboard.com, hverkuil@...all.nl,
	kieran.bingham@...asonboard.com, naush@...pberrypi.com,
	mchehab@...nel.org, hdegoede@...hat.com,
	dave.stevenson@...pberrypi.com, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3] media: v4l2-common: Add a helper for obtaining
 the clock producer

Hi Mehdi,

Thank you for the patch.

On Fri, Mar 21, 2025 at 10:38:14AM +0100, Mehdi Djait wrote:
> Introduce a helper for v4l2 sensor drivers on both DT- and ACPI-based
> platforms to retrieve a reference to the clock producer from firmware.
> 
> This helper behaves the same as clk_get_optional() except where there is
> no clock producer like in ACPI-based platforms.
> 
> For ACPI-based platforms the function will read the "clock-frequency"
> ACPI _DSD property and register a fixed frequency clock with the frequency
> indicated in the property.
> 
> Signed-off-by: Mehdi Djait <mehdi.djait@...ux.intel.com>
> ---
> Link for discussion (where this patch was proposed): https://lore.kernel.org/linux-media/20250220154909.152538-1-mehdi.djait@linux.intel.com/
> 
> v1 -> v2:
> Suggested by Sakari:
>     - removed clk_name
>     - removed the IS_ERR() check
>     - improved the kernel-doc comment and commit msg
> Link for v1: https://lore.kernel.org/linux-media/20250227092643.113939-1-mehdi.djait@linux.intel.com
> 
> v2 -> v3:
> - Added #ifdef CONFIG_COMMON_CLK for the ACPI case
> 
>  drivers/media/v4l2-core/v4l2-common.c | 39 +++++++++++++++++++++++++++
>  include/media/v4l2-common.h           | 18 +++++++++++++
>  2 files changed, 57 insertions(+)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-common.c b/drivers/media/v4l2-core/v4l2-common.c
> index 0a2f4f0d0a07..4e30f8b777b7 100644
> --- a/drivers/media/v4l2-core/v4l2-common.c
> +++ b/drivers/media/v4l2-core/v4l2-common.c
> @@ -34,6 +34,9 @@
>   * Added Gerd Knorrs v4l1 enhancements (Justin Schoeman)
>   */
>  
> +#include <linux/clk.h>
> +#include <linux/clkdev.h>
> +#include <linux/clk-provider.h>
>  #include <linux/module.h>
>  #include <linux/types.h>
>  #include <linux/kernel.h>
> @@ -636,3 +639,39 @@ int v4l2_link_freq_to_bitmap(struct device *dev, const u64 *fw_link_freqs,
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(v4l2_link_freq_to_bitmap);
> +
> +struct clk *devm_v4l2_sensor_clk_get(struct device *dev, const char *id)
> +{
> +	struct clk_hw *clk_hw;
> +	struct clk *clk;
> +	u32 rate;
> +	int ret;
> +
> +	clk = devm_clk_get_optional(dev, id);
> +	if (clk)
> +		return clk;
> +
> +#ifdef CONFIG_COMMON_CLK

This patch will cause warnings when CONFIG_COMMON_CLK is disabled. Could
you use

	if (IS_REACHABLE(CONFIG_COMMON_CLK)) {
		...
	}

instead ? It will also ensure that all code gets compile-tested, even
when CONFIG_COMMON_CLK is disabled ?

If you want to minimize implementation, you could write

	if (!IS_REACHABLE(CONFIG_COMMON_CLK))
		return ERR_PTR(-ENOENT);

and keep the code below as-is.

> +	if (!is_acpi_node(dev_fwnode(dev)))
> +		return ERR_PTR(-ENOENT);
> +
> +	ret = device_property_read_u32(dev, "clock-frequency", &rate);
> +	if (ret)
> +		return ERR_PTR(ret);
> +
> +	if (!id) {
> +		id = devm_kasprintf(dev, GFP_KERNEL, "clk-%s", dev_name(dev));

As far as I understand, the name doesn't need to stay valid after
devm_clk_hw_register_fixed_rate() returns. You can call kasprintf here,
and call kfree after devm_clk_hw_register_fixed_rate(). You could use
__free to manage the memory life time:

	const char *clk_id __free(kfree) = NULL;

	if (!id) {
		clk_id = kasprintf(GFP_KERNEL, "clk-%s", dev_name(dev));
		if (!clk_id)
			return ERR_PTR(-ENOMEM);
		id = clk_id;
	}

> +		if (!id)
> +			return ERR_PTR(-ENOMEM);
> +	}
> +
> +	clk_hw = devm_clk_hw_register_fixed_rate(dev, id, NULL, 0, rate);
> +	if (IS_ERR(clk_hw))
> +		return ERR_CAST(clk_hw);
> +
> +	return clk_hw->clk;
> +#else
> +	return ERR_PTR(-ENOENT);
> +#endif
> +}
> +EXPORT_SYMBOL_GPL(devm_v4l2_sensor_clk_get);
> diff --git a/include/media/v4l2-common.h b/include/media/v4l2-common.h
> index 63ad36f04f72..35b9ac698e8a 100644
> --- a/include/media/v4l2-common.h
> +++ b/include/media/v4l2-common.h
> @@ -573,6 +573,24 @@ int v4l2_link_freq_to_bitmap(struct device *dev, const u64 *fw_link_freqs,
>  			     unsigned int num_of_driver_link_freqs,
>  			     unsigned long *bitmap);
>  
> +/**
> + * devm_v4l2_sensor_clk_get - lookup and obtain a reference to an optional clock
> + *			      producer for a camera sensor.
> + *
> + * @dev: device for v4l2 sensor clock "consumer"
> + * @id: clock consumer ID
> + *
> + * This function behaves the same way as clk_get_optional() except where there
> + * is no clock producer like in ACPI-based platforms.
> + * For ACPI-based platforms, the function will read the "clock-frequency"
> + * ACPI _DSD property and register a fixed-clock with the frequency indicated
> + * in the property.
> + *
> + * Return:
> + * * pointer to a struct clk on success or an error code on failure.
> + */
> +struct clk *devm_v4l2_sensor_clk_get(struct device *dev, const char *id);
> +
>  static inline u64 v4l2_buffer_get_timestamp(const struct v4l2_buffer *buf)
>  {
>  	/*

-- 
Regards,

Laurent Pinchart