linux-kernel - Re: [RFC PATCH v2] media: v4l2-common: Add a helper for obtaining the clock producer

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Message-ID: <urd3bng2yixuiny536imfoihhe6uyowynih7gkc4q6pkr6mijy@ggqlz5zu5isf>
Date: Mon, 12 May 2025 10:21:21 +0200
From: Mehdi Djait <mehdi.djait@...ux.intel.com>
To: Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc: laurent.pinchart@...asonboard.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 v2] media: v4l2-common: Add a helper for obtaining
 the clock producer

Hi Sakari,

On Sat, May 10, 2025 at 12:56:02PM +0000, Sakari Ailus wrote:
> Hi Mehdi,
> 
> On Mon, Mar 10, 2025 at 01:23:05PM +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 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>

SNIP

> > +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;
> > +
> > +	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));
> > +		if (!id)
> > +			return ERR_PTR(-ENOMEM);
> > +	}
> > +
> > +	clk_hw = devm_clk_hw_register_fixed_rate(dev, id, NULL, 0, rate);
> 
> devm_clk_hw_register_fixed_rate() is only available when COMMON_CLK is
> enabled. You need #ifdefs here. In practice without CCF only
> devm_clk_get_optional() is useful I guess.
> 

I added a call to IS_REACHABLE(CONFIG_COMMON_CLK) in the v4 of this patch:
https://lore.kernel.org/linux-media/20250321130329.342236-1-mehdi.djait@linux.intel.com/

> Another question is then how commonly COMMON_CLK is enabled e.g. on x86
> systems. At least Debian kernel has it. Presumably it's common elsewhere,
> too.

on Arch linux it is also enabled and Fedora also. I would also assume it
is also enabled in the other linux distros.

--
Kind Regards
Mehdi Djait