Re: [PATCH v3 1/7] driver core: auxiliary bus: add device creation helpers

[Jerome Brunet <jbrunet@baylibre.com>]

On Fri 14 Feb 2025 at 17:33, Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:

> On Tue, Feb 11, 2025 at 06:27:58PM +0100, Jerome Brunet wrote:
>> Add helper functions to create a device on the auxiliary bus.
>> 
>> This is meant for fairly simple usage of the auxiliary bus, to avoid having
>> the same code repeated in the different drivers.
>> 
>> Suggested-by: Stephen Boyd <sboyd@kernel.org>
>> Cc: Arnd Bergmann <arnd@arndb.de>
>> Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
>> ---
>>  drivers/base/auxiliary.c      | 88 +++++++++++++++++++++++++++++++++++++++++++
>>  include/linux/auxiliary_bus.h | 10 +++++
>>  2 files changed, 98 insertions(+)
>
> I like the idea, see much the same of what I recently did for the "faux"
> bus here:
> 	https://lore.kernel.org/all/2025021023-sandstorm-precise-9f5d@gregkh/

Reading this, I'm getting the feeling that some (most?) simple auxiliary
driver might be better off migrating to "faux", instead of what I'm
proposing here ? Is this what you are suggesting ?

Few Q:
Is there some sort of 'platform_data' (sorry for the lack of a better
term, no provocation intended ;) ) ... it there a
simple way to pass an arbitrary struct to the created device with 'faux' ?

The difference between aux and faux I'm seeing it that aux seems to
decouple things a bit more. The only thing aux needs is a module name to
pop something up, while faux needs a reference to the ops instead.

I can see the appeal to use aux for maintainers trying to decouple
different subsystems.

>
> Some review comments:
>
>> diff --git a/drivers/base/auxiliary.c b/drivers/base/auxiliary.c
>> index afa4df4c5a3f371b91d8dd8c4325495d32ad1291..0f697c9c243dc9a50498a52362806db594345faf 100644
>> --- a/drivers/base/auxiliary.c
>> +++ b/drivers/base/auxiliary.c
>> @@ -385,6 +385,94 @@ void auxiliary_driver_unregister(struct auxiliary_driver *auxdrv)
>>  }
>>  EXPORT_SYMBOL_GPL(auxiliary_driver_unregister);
>>  
>> +static void auxiliary_device_release(struct device *dev)
>> +{
>> +	struct auxiliary_device *auxdev = to_auxiliary_dev(dev);
>> +
>> +	kfree(auxdev);
>> +}
>> +
>> +static struct auxiliary_device *auxiliary_device_create(struct device *dev,
>> +							const char *modname,
>> +							const char *devname,
>> +							void *platform_data,
>
> Can you have the caller set the platform_data if they need/want it after
> the device is created?  Or do you need that in the probe callback?

My assumption was that it is needed in probe, but I guess that entirely
depends on the driver. If that was ever needed, it could be added later
I think.

>
> And can't this be a global function too for those that don't want to
> deal with devm stuff?

There was a note about that in the cover-letter of the v1 but I did not
repeat it after.

It can be exported but I had no use for it so I thought It was better not
export it until it was actually needed. I really do not have a strong
preference over this.

>
>> +							int id)
>> +{
>> +	struct auxiliary_device *auxdev;
>> +	int ret;
>> +
>> +	auxdev = kzalloc(sizeof(*auxdev), GFP_KERNEL);
>> +	if (!auxdev)
>> +		return ERR_PTR(-ENOMEM);
>
> Ick, who cares what the error value really is?  Why not just do NULL or
> a valid pointer?  That makes the caller much simpler to handle, right?
>

Sure why not

>> +
>> +	auxdev->id = id;
>> +	auxdev->name = devname;
>> +	auxdev->dev.parent = dev;
>> +	auxdev->dev.platform_data = platform_data;
>> +	auxdev->dev.release = auxiliary_device_release;
>> +	device_set_of_node_from_dev(&auxdev->dev, dev);
>> +
>> +	ret = auxiliary_device_init(auxdev);
>
> Only way this will fail is if you forgot to set parent or a valid name.
> So why not check for devname being non-NULL above this?

If auxiliary_device_init() ever changes it would be easy to forget to
update that and lead to something nasty to debug, don't you think ?

If you are OK with this, I could update in this direction.

>
>> +	if (ret) {
>> +		kfree(auxdev);
>> +		return ERR_PTR(ret);
>> +	}
>> +
>> +	ret = __auxiliary_device_add(auxdev, modname);
>> +	if (ret) {
>> +		/*
>> +		 * NOTE: It may look odd but auxdev should not be freed
>> +		 * here. auxiliary_device_uninit() calls device_put()
>> +		 * which call the device release function, freeing auxdev.
>> +		 */
>> +		auxiliary_device_uninit(auxdev);
>
> Yes it is odd, are you SURE you should be calling device_del() on the
> device if this fails?  auxiliary_device_uninit(), makes sense so why not
> just call that here?

I'm confused ... I am call auxiliary_device_uninit() here. What do you
mean ? 

>
>> +		return ERR_PTR(ret);
>> +	}
>> +
>> +	return auxdev;
>> +}
>> +
>> +static void auxiliary_device_destroy(void *_auxdev)
>> +{
>> +	struct auxiliary_device *auxdev = _auxdev;
>> +
>> +	auxiliary_device_delete(auxdev);
>> +	auxiliary_device_uninit(auxdev);
>> +}
>> +
>> +/**
>> + * __devm_auxiliary_device_create - create a device on the auxiliary bus
>> + * @dev: parent device
>> + * @modname: module name used to create the auxiliary driver name.
>> + * @devname: auxiliary bus device name
>> + * @platform_data: auxiliary bus device platform data
>> + * @id: auxiliary bus device id
>> + *
>> + * Device managed helper to create an auxiliary bus device.
>> + * The device create matches driver 'modname.devname' on the auxiliary bus.
>> + */
>> +struct auxiliary_device *__devm_auxiliary_device_create(struct device *dev,
>> +							const char *modname,
>> +							const char *devname,
>> +							void *platform_data,
>> +							int id)
>> +{
>> +	struct auxiliary_device *auxdev;
>> +	int ret;
>> +
>> +	auxdev = auxiliary_device_create(dev, modname, devname, platform_data, id);
>> +	if (IS_ERR(auxdev))
>> +		return auxdev;
>> +
>> +	ret = devm_add_action_or_reset(dev, auxiliary_device_destroy,
>> +				       auxdev);
>
> Oh this is going to be messy, but I trust that callers know what they
> are doing here.  Good luck!  :)
>
> thanks,
>
> greg k-h

-- 
Jerome

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