netdev - Re: [PATCH v2 1/6] Add ancillary bus support

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Date:   Tue, 6 Oct 2020 10:18:07 -0500
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Leon Romanovsky <leon@...nel.org>,
        Dave Ertman <david.m.ertman@...el.com>
Cc:     alsa-devel@...a-project.org, parav@...lanox.com, tiwai@...e.de,
        netdev@...r.kernel.org, ranjani.sridharan@...ux.intel.com,
        fred.oh@...ux.intel.com, linux-rdma@...r.kernel.org,
        dledford@...hat.com, broonie@...nel.org, jgg@...dia.com,
        gregkh@...uxfoundation.org, kuba@...nel.org,
        dan.j.williams@...el.com, shiraz.saleem@...el.com,
        davem@...emloft.net, kiran.patil@...el.com
Subject: Re: [PATCH v2 1/6] Add ancillary bus support

Thanks for the review Leon.

>> Add support for the Ancillary Bus, ancillary_device and ancillary_driver.
>> It enables drivers to create an ancillary_device and bind an
>> ancillary_driver to it.
> 
> I was under impression that this name is going to be changed.

It's part of the opens stated in the cover letter.

[...]

>> +	const struct my_driver my_drv = {
>> +		.ancillary_drv = {
>> +			.driver = {
>> +				.name = "myancillarydrv",
> 
> Why do we need to give control over driver name to the driver authors?
> It can be problematic if author puts name that already exists.

Good point. When I used the ancillary_devices for my own SoundWire test, 
the driver name didn't seem specifically meaningful but needed to be set 
to something, what mattered was the id_table. Just thinking aloud, maybe 
we can add prefixing with KMOD_BUILD, as we've done already to avoid 
collisions between device names?

[...]

>> +int __ancillary_device_add(struct ancillary_device *ancildev, const char *modname)
>> +{
>> +	struct device *dev = &ancildev->dev;
>> +	int ret;
>> +
>> +	if (!modname) {
>> +		pr_err("ancillary device modname is NULL\n");
>> +		return -EINVAL;
>> +	}
>> +
>> +	ret = dev_set_name(dev, "%s.%s.%d", modname, ancildev->name, ancildev->id);
>> +	if (ret) {
>> +		pr_err("ancillary device dev_set_name failed: %d\n", ret);
>> +		return ret;
>> +	}
>> +
>> +	ret = device_add(dev);
>> +	if (ret)
>> +		dev_err(dev, "adding ancillary device failed!: %d\n", ret);
>> +
>> +	return ret;
>> +}
> 
> Sorry, but this is very strange API that requires users to put
> internal call to "dev" that is buried inside "struct ancillary_device".
> 
> For example in your next patch, you write this "put_device(&cdev->ancildev.dev);"
> 
> I'm pretty sure that the amount of bugs in error unwind will be
> astonishing, so if you are doing wrappers over core code, better do not
> pass complexity to the users.

In initial reviews, there was pushback on adding wrappers that don't do 
anything except for a pointer indirection.

Others had concerns that the API wasn't balanced and blurring layers.

Both points have merits IMHO. Do we want wrappers for everything and 
completely hide the low-level device?

> 
>> +EXPORT_SYMBOL_GPL(__ancillary_device_add);
>> +
>> +static int ancillary_probe_driver(struct device *dev)
>> +{
>> +	struct ancillary_driver *ancildrv = to_ancillary_drv(dev->driver);
>> +	struct ancillary_device *ancildev = to_ancillary_dev(dev);
>> +	int ret;
>> +
>> +	ret = dev_pm_domain_attach(dev, true);
>> +	if (ret) {
>> +		dev_warn(dev, "Failed to attach to PM Domain : %d\n", ret);
>> +		return ret;
>> +	}
>> +
>> +	ret = ancildrv->probe(ancildev, ancillary_match_id(ancildrv->id_table, ancildev));
> 
> I don't think that you need to call ->probe() if ancillary_match_id()
> returned NULL and probably that check should be done before
> dev_pm_domain_attach().

we'll look into this.

> 
>> +	if (ret)
>> +		dev_pm_domain_detach(dev, true);
>> +
>> +	return ret;
>> +}
>> +
>> +static int ancillary_remove_driver(struct device *dev)
>> +{
>> +	struct ancillary_driver *ancildrv = to_ancillary_drv(dev->driver);
>> +	struct ancillary_device *ancildev = to_ancillary_dev(dev);
>> +	int ret;
>> +
>> +	ret = ancildrv->remove(ancildev);
>> +	dev_pm_domain_detach(dev, true);
>> +
>> +	return ret;
> 
> You returned an error to user and detached from PM, what will user do
> with this information? Should user ignore it? retry?

That comment was also provided in earlier reviews. In practice the error 
is typically ignored so there was a suggestion to move the return type 
to void, that could be done if this was desired by the majority.

[...]

>> diff --git a/include/linux/mod_devicetable.h b/include/linux/mod_devicetable.h
>> index 5b08a473cdba..7d596dc30833 100644
>> --- a/include/linux/mod_devicetable.h
>> +++ b/include/linux/mod_devicetable.h
>> @@ -838,4 +838,12 @@ struct mhi_device_id {
>>   	kernel_ulong_t driver_data;
>>   };
>>
>> +#define ANCILLARY_NAME_SIZE 32
>> +#define ANCILLARY_MODULE_PREFIX "ancillary:"
>> +
>> +struct ancillary_device_id {
>> +	char name[ANCILLARY_NAME_SIZE];
> 
> I hope that this be enough.

Are you suggesting a different value to allow for a longer string?