linux-kernel - Re: [PATCH v2] hwmon: corsair-cpro: Change to HID driver

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Date:   Wed, 15 Jul 2020 21:55:36 +0200
From:   Marius Zachmann <mail@...iuszachmann.de>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Jean Delvare <jdelvare@...e.com>, linux-hwmon@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] hwmon: corsair-cpro: Change to HID driver

On 15.07.20 at 21:49:59 CEST, Guenter Roeck wrote
> On 7/15/20 12:12 PM, Marius Zachmann wrote:
> > On 15.07.20 at 17:35:08 CEST, Guenter Roeck wrote
> >> On 7/15/20 8:14 AM, Marius Zachmann wrote:
> >>> This changes corsair-cpro to a hid driver using hid reports.
> >>>
> >>> Signed-off-by: Marius Zachmann <mail@...iuszachmann.de>
> >>>
> >>> ---
> >>
> > ...
> >>> -module_usb_driver(ccp_driver);
> >>> +static int __init ccp_init(void)
> >>> +{
> >>> +	return hid_register_driver(&ccp_driver);
> >>> +}
> >>> +
> >>> +static void __exit ccp_exit(void)
> >>> +{
> >>> +	hid_unregister_driver(&ccp_driver);
> >>> +}
> >>> +
> >>> +/* make sure it is loaded after hid */
> >>> +late_initcall(ccp_init);
> >>> +module_exit(ccp_exit);
> >>
> >> Sorry for not noticing before, but can you use module_hid_driver() ?
> >> That seems to work for other hid drivers.
> >>
> >> Thanks,
> >> Guenter
> >>
> > 
> > When using module_hid_driver() and compiling the driver as built-in
> > the driver init function will get called before hid and fail registering
> > the driver. late_initcall forces it to be loaded after hid.
> > 
> 
> That seems odd. Why does no other hid driver have that problem ?
> Your argument is pretty much that module_hid_driver() would not work,
> which seems unlikely.
> 
> Thanks,
> Guenter
> 

The order of initcalls is determined by the build order. Hwmon subsystem
gets build before hid subsystem and therefore the initcalls are earlier
than hid. But hid_register_driver needs the hid driver to be there.

Greetings
Marius