RE: [PATCH v2 1/6] Add ancillary bus support

["Ertman, David M" <david.m.ertman@intel.com>]

> -----Original Message-----
> From: Leon Romanovsky <leon@kernel.org>
> Sent: Tuesday, October 6, 2020 10:03 AM
> To: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
> Cc: Ertman, David M <david.m.ertman@intel.com>; alsa-devel@alsa-
> project.org; parav@mellanox.com; tiwai@suse.de; netdev@vger.kernel.org;
> ranjani.sridharan@linux.intel.com; fred.oh@linux.intel.com; linux-
> rdma@vger.kernel.org; dledford@redhat.com; broonie@kernel.org;
> jgg@nvidia.com; gregkh@linuxfoundation.org; kuba@kernel.org; Williams,
> Dan J <dan.j.williams@intel.com>; Saleem, Shiraz
> <shiraz.saleem@intel.com>; davem@davemloft.net; Patil, Kiran
> <kiran.patil@intel.com>
> Subject: Re: [PATCH v2 1/6] Add ancillary bus support
> 
> On Tue, Oct 06, 2020 at 10:18:07AM -0500, Pierre-Louis Bossart wrote:
> > 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.
> 
> ok, so what are the variants?
> system bus (sysbus), sbsystem bus (subbus), crossbus ?
> 
> >
> > [...]
> >
> > > > +	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?
> 
> IMHO, it shouldn't be controlled by the drivers at all and need to have
> kernel module name hardwired. Users will use it later for various
> bind/unbind/autoprobe tricks and it will give predictability for them.
> 
> >
> > [...]
> >
> > > > +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.
> 
> Are you talking about internal review or public?
> If it is public, can I get a link to it?
> 
> >
> > Both points have merits IMHO. Do we want wrappers for everything and
> > completely hide the low-level device?
> 
> This API is partially obscures low level driver-core code and needs to
> provide clear and proper abstractions without need to remember about
> put_device. There is already _add() interface why don't you do
> put_device() in it?
> 

The pushback Pierre is referring to was during our mid-tier internal review.  It was
primarily a concern of Parav as I recall, so he can speak to his reasoning.

What we originally had was a single API call (ancillary_device_register) that started
with a call to device_initialize(), and every error path out of the function performed
a put_device().

Is this the model you have in mind?

-DaveE

> >
> > >
> > > > +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.
> 
> +1 from me.
> 
> >
> > [...]
> >
> > > > 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?
> 
> I have no idea, but worried that there were no checks at all if name is
> more than 32. Maybe compiler warn about it?
> 
> Thanks