[RFC PATCH v4] media: v4l2-common: Add a helper for obtaining the clock producer

[Mehdi Djait <mehdi.djait@linux.intel.com>]

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@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 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
Link v2: https://lore.kernel.org/linux-media/20250310122305.209534-1-mehdi.djait@linux.intel.com/

v3 -> v4:
Suggested by Laurent:
	- removed the #ifdef to use IS_REACHABLE(CONFIG_COMMON_CLK)
	- changed to kasprintf() to allocate the clk name when id is NULL and
	  used the __free(kfree) scope-based cleanup helper when
	  defining the variable to hold the allocated name
Link v3: https://lore.kernel.org/linux-media/20250321093814.18159-1-mehdi.djait@linux.intel.com/


 drivers/media/v4l2-core/v4l2-common.c | 40 +++++++++++++++++++++++++++
 include/media/v4l2-common.h           | 18 ++++++++++++
 2 files changed, 58 insertions(+)

diff --git a/drivers/media/v4l2-core/v4l2-common.c b/drivers/media/v4l2-core/v4l2-common.c
index 0a2f4f0d0a07..b33152e2c3af 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 +639,40 @@ 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)
+{
+	const char *clk_id __free(kfree) = NULL;
+	struct clk_hw *clk_hw;
+	struct clk *clk;
+	u32 rate;
+	int ret;
+
+	clk = devm_clk_get_optional(dev, id);
+	if (clk)
+		return clk;
+
+	if (!IS_REACHABLE(CONFIG_COMMON_CLK))
+		return ERR_PTR(-ENOENT);
+
+	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) {
+		clk_id = kasprintf(GFP_KERNEL, "clk-%s", dev_name(dev));
+		if (!clk_id)
+			return ERR_PTR(-ENOMEM);
+		id = clk_id;
+	}
+
+	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;
+}
+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)
 {
 	/*
-- 
2.48.1