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

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

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);
 
 /*
  * 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");
 MODULE_LICENSE("GPL");
-- 
2.47.0