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

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Message-ID: <dumi72cs5fc423kidnmbtxgbsbir5r3f5u4pyzola7wvznyh2l@4otjmv2qaqjv>
Date: Wed, 14 May 2025 13:43:03 +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, 
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [RFC PATCH v2] media: v4l2-common: Add a helper for obtaining
 the clock producer

Hi Sakari,

On Mon, May 12, 2025 at 06:41:07PM +0300, Sakari Ailus wrote:
> Hi Mehdi,
> 
> On Mon, May 12, 2025 at 10:21:21AM +0200, Mehdi Djait wrote:
> > 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/
> 
> I wonder if this approach works. Depending on the compiler implementation,
> the compiler could (or even should) still run into issues in finding an
> unresolvable symbol, even if the symbol is not reachable and can be
> optimised away.
> 

So I discussed with Arnd about this (He introduced IS_REACHABLE()):

- IS_REACHABLE() is actually discouraged [1]

- COFIG_COMMON_CLK is a bool, so IS_ENABLED() will be the right solution here

- usage of IS_ENABLED() is also encouraged in the coding style [2]

- we will not face compiler issues because the kernel relies on
  dead code elimination in the compiler. (Actually I remember this is
  one of the reasons why you cannot compile the kernel with optimization
  turned off. I don't know how much this argument holds today [3]

[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
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/coding-style.rst#n1178
[3] https://lore.kernel.org/all/20080909160452.GB30160@cs181140183.pp.htv.fi/

> > 
> > > 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.
> 
> Ack, thanks for checking.
> 
> -- 
> Regards,
> 
> Sakari Ailus

--
Kind Regards
Mehdi Djait