[RFC PATCH v5] 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.

This function also handles the special ACPI-based system case where:
The clock-frequency _DSD property is present.
A reference to the clock producer is present, where the clock is provided
by a camera sensor PMIC driver (e.g. int3472/tps68470.c).
In this case try to set the clock-frequency value to the provided clock.

Signed-off-by: Mehdi Djait <mehdi.djait@linux.intel.com>
---
v4 -> v5:
Suggested by Arnd Bergmann:
	- removed IS_REACHABLE(CONFIG_COMMON_CLK). IS_REACHABLE() is actually
	discouraged [1]. COFIG_COMMON_CLK is a bool, so IS_ENABLED() will be the
	right solution here
Suggested by Hans de Goede:
	- added handling for the special ACPI-based system case, where
	  both a reference to the clock-provider and the _DSD
	  clock-frequency are present.
	- updated the function's kernel-doc and the commit msg
	  to mention this special case.
Link v4: https://lore.kernel.org/linux-media/20250321130329.342236-1-mehdi.djait@linux.intel.com/
[1] https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/Documentation/kbuild/kconfig-language.rst?h=next-20250513&id=700bd25bd4f47a0f4e02e0a25dde05f1a6b16eea

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/

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/

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



 drivers/media/v4l2-core/v4l2-common.c | 46 +++++++++++++++++++++++++++
 include/media/v4l2-common.h           | 25 +++++++++++++++
 2 files changed, 71 insertions(+)

diff --git a/drivers/media/v4l2-core/v4l2-common.c b/drivers/media/v4l2-core/v4l2-common.c
index 4ee4aa19efe6..6099acd339ad 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>
@@ -665,3 +668,46 @@ 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);
+	ret = device_property_read_u32(dev, "clock-frequency", &rate);
+
+	if (clk) {
+		if (!ret) {
+			ret = clk_set_rate(clk, rate);
+			if (ret)
+				dev_warn(dev, "Failed to set clock rate: %u\n",
+					 rate);
+		}
+
+		return clk;
+	}
+
+	if (ret)
+		return ERR_PTR(ret);
+
+	if (!IS_ENABLED(CONFIG_COMMON_CLK) || !is_acpi_node(dev_fwnode(dev)))
+		return ERR_PTR(-ENOENT);
+
+	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 fda903bb3674..5ddbf7b3d9c3 100644
--- a/include/media/v4l2-common.h
+++ b/include/media/v4l2-common.h
@@ -586,6 +586,31 @@ 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.
+ *
+ * This function also handles the special ACPI-based system case where:
+ * The clock-frequency _DSD property is present.
+ * A reference to the clock producer is present, where the clock is provided by
+ * a camera sensor PMIC driver (e.g. int3472/tps68470.c)
+ * In this case try to set the clock-frequency value to the provided clock.
+ *
+ * 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)
 {
 	/*
-- 
2.49.0