linux-kernel - Re: [RFC PATCH] cpufreq: dt-platdev: Fix module autoloading

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Message-ID: <20241121071127.y66uoamjmroukjck@vireshk-i7>
Date: Thu, 21 Nov 2024 12:41:27 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: Javier Martinez Canillas <javierm@...hat.com>, robh@...nel.org,
	arnd@...aro.org
Cc: linux-kernel@...r.kernel.org, Radu Rendec <rrendec@...hat.com>,
	Zhipeng Wang <zhipeng.wang_1@....com>,
	Maxime Ripard <mripard@...nel.org>, javier@...hile0.org,
	"Rafael J. Wysocki" <rafael@...nel.org>, linux-pm@...r.kernel.org
Subject: Re: [RFC PATCH] cpufreq: dt-platdev: Fix module autoloading

+Rob/Arnd,

On 19-11-24, 12:18, Javier Martinez Canillas wrote:
> This driver can be built as a module since commit 3b062a086984 ("cpufreq:
> dt-platdev: Support building as module"), but unfortunately this caused
> a regression because the cputfreq-dt-platdev.ko module does not autoload.
> 
> Usually, this is solved by just using the MODULE_DEVICE_TABLE() macro to
> export all the device IDs as module aliases. But this driver is special
> due how matches with devices and decides what platform supports.
> 
> There are two of_device_id lists, an allow list that are for CPU devices
> that always match and a deny list that's for devices that must not match.
> 
> The driver registers a cpufreq-dt platform device for all the CPU device
> nodes that either are in the allow list or contain an operating-points-v2
> property and are not in the deny list.
> 
> For the former just add a MODULE_DEVICE_TABLE(), and for the latter add a
> module alias. That way the driver would always be autoloaded when needed.
> 
> Reported-by: Radu Rendec <rrendec@...hat.com>
> Fixes: 3b062a086984 ("cpufreq: dt-platdev: Support building as module")
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
> ---
> Posting as an RFC because I don't have a platform that uses this driver
> but I'll let Radu test since he reported by issue.
> 
>  drivers/cpufreq/cpufreq-dt-platdev.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/cpufreq/cpufreq-dt-platdev.c b/drivers/cpufreq/cpufreq-dt-platdev.c
> index 2a3e8bd317c9..7ae7c897c249 100644
> --- a/drivers/cpufreq/cpufreq-dt-platdev.c
> +++ b/drivers/cpufreq/cpufreq-dt-platdev.c
> @@ -97,6 +97,7 @@ static const struct of_device_id allowlist[] __initconst = {
>  
>  	{ }
>  };
> +MODULE_DEVICE_TABLE(of, allowlist);

This is fine obviously.

>  /*
>   * Machines for which the cpufreq device is *not* created, mostly used for
> @@ -236,4 +237,16 @@ static int __init cpufreq_dt_platdev_init(void)
>  }
>  core_initcall(cpufreq_dt_platdev_init);
>  MODULE_DESCRIPTION("Generic DT based cpufreq platdev driver");
> +/*
> + * The module alias is needed because the driver automatically registers a
> + * platform device for any CPU device node that has an operating-points-v2
> + * property and is not in the block list.
> + *
> + * For this reason the MODULE_DEVICE_TABLE() macro can only export aliases
> + * of the devices in the allow list, which means that the driver will not
> + * autoload for devices whose cpufreq-dt will be registered automatically.
> + *
> + * Adding an "of:N*T*Coperating-points-v2" alias is a workaround for this.
> + */
> +MODULE_ALIAS("of:N*T*Coperating-points-v2");

How does this work? This will autoload the module for any platform with
"operating-points-v2" property in the DT ? The driver though works only if the
property is in the CPU node, while now we will try to load this driver even if a
non-CPU node has the property now.

I am not sure what's the best way forward to fix this.

Arnd, Rob, any inputs ?

>  MODULE_LICENSE("GPL");

-- 
viresh