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

[Javier Martinez Canillas <javierm@redhat.com>]

Viresh Kumar <viresh.kumar@linaro.org> writes:

Hello Viresh,

> +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@redhat.com>
>> Fixes: 3b062a086984 ("cpufreq: dt-platdev: Support building as module")
>> Signed-off-by: Javier Martinez Canillas <javierm@redhat.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.
>

Yeah, this part is trivial.

>>  /*
>>   * 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.
>

Will autload the driver for any platform that has a Device Tree node with a
compatible = "operating-points-v2" (assuming that this node will be a phandle
for the "operating-points-v2" property.

For example, in the case of the following DT snippet:

cpus {
        cpu@0 {
                operating-points-v2     = <&cpu0_opp_table>;
        };
};

cpu0_opp_table: opp_table {
        compatible = "operating-points-v2";
...
};

It will autoload if OF finds the opp_table node, but it register the cpufreq-dt
device only if there's a cpu@0 with a "operating-points-v2" property.

Yes, there may be false positives because the autload semantics don't exactly
match the criteria for the driver to "match" but I believe is better to load it
and not use for those cases, than needing the driver and not autoloading it.

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

I couldn't find another way to solve it, if you have a better idea please let
me know. But IMO we should either workaround like this or revert the commit 
that changed the driver's Kconfig symbol to be tristate.

> Arnd, Rob, any inputs ?
>
>>  MODULE_LICENSE("GPL");
>
> -- 
> viresh
>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat