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

From: "Ertman, David M" <david.m.ertman@intel.com>
To: Leon Romanovsky <leon@kernel.org>
Cc: "alsa-devel@alsa-project.org" <alsa-devel@alsa-project.org>,
	"parav@mellanox.com" <parav@mellanox.com>,
	"tiwai@suse.de" <tiwai@suse.de>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"ranjani.sridharan@linux.intel.com" 
	<ranjani.sridharan@linux.intel.com>,
	Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>,
	"fred.oh@linux.intel.com" <fred.oh@linux.intel.com>,
	"linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>,
	"dledford@redhat.com" <dledford@redhat.com>,
	"broonie@kernel.org" <broonie@kernel.org>,
	"jgg@nvidia.com" <jgg@nvidia.com>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	"kuba@kernel.org" <kuba@kernel.org>,
	"Williams, Dan J" <dan.j.williams@intel.com>,
	"Saleem, Shiraz" <shiraz.saleem@intel.com>,
	"davem@davemloft.net" <davem@davemloft.net>,
	"Patil, Kiran" <kiran.patil@intel.com>
Subject: RE: [PATCH v2 1/6] Add ancillary bus support
Date: Wed, 7 Oct 2020 19:53:40 +0000	[thread overview]
Message-ID: <DM6PR11MB2841EBB5C27A947789F9DB73DD0A0@DM6PR11MB2841.namprd11.prod.outlook.com> (raw)
In-Reply-To: <20201007192610.GD3964015@unreal>

> -----Original Message-----
> From: Alsa-devel <alsa-devel-bounces@alsa-project.org> On Behalf Of Leon
> Romanovsky
> Sent: Wednesday, October 7, 2020 12:26 PM
> To: Ertman, David M <david.m.ertman@intel.com>
> Cc: alsa-devel@alsa-project.org; parav@mellanox.com; tiwai@suse.de;
> netdev@vger.kernel.org; ranjani.sridharan@linux.intel.com; Pierre-Louis
> Bossart <pierre-louis.bossart@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 Wed, Oct 07, 2020 at 06:06:30PM +0000, Ertman, David M wrote:
> > > -----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?
> 
> I don't like this flow:
> ancillary_device_initialize()
> if (ancillary_ancillary_device_add()) {
>   put_device(....)
>   ancillary_device_unregister()
>   return err;
> }
> 
> And prefer this flow:
> ancillary_device_initialize()
> if (ancillary_device_add()) {
>   ancillary_device_unregister()
>   return err;
> }
> 
> In this way, the ancillary users won't need to do non-intuitive put_device();

Isn't there a problem calling device_unregister() if device_add() fails?
device_unregister() does a device_del() and if the device_add() failed there is
nothing to delete?

-DaveE

> 
> Thanks