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

[Leon Romanovsky <leon@kernel.org>]

On Wed, Oct 07, 2020 at 08:46:45PM +0000, Ertman, David M wrote:
> > -----Original Message-----
> > From: Parav Pandit <parav@nvidia.com>
> > Sent: Wednesday, October 7, 2020 1:17 PM
> > To: Leon Romanovsky <leon@kernel.org>; Ertman, David M
> > <david.m.ertman@intel.com>
> > Cc: Pierre-Louis Bossart <pierre-louis.bossart@linux.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; Jason Gunthorpe
> > <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
> >
> >
> > > From: Leon Romanovsky <leon@kernel.org>
> > > Sent: Thursday, October 8, 2020 12:56 AM
> > >
> > > > > 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()
> > Calling device_unregister() is incorrect, because add() wasn't successful.
> > Only put_device() or a wrapper ancillary_device_put() is necessary.
> >
> > >   return err;
> > > }
> > >
> > > And prefer this flow:
> > > ancillary_device_initialize()
> > > if (ancillary_device_add()) {
> > >   ancillary_device_unregister()
> > This is incorrect and a clear deviation from the current core APIs that adds the
> > confusion.
> >
> > >   return err;
> > > }
> > >
> > > In this way, the ancillary users won't need to do non-intuitive put_device();
> >
> > Below is most simple, intuitive and matching with core APIs for name and
> > design pattern wise.
> > init()
> > {
> > 	err = ancillary_device_initialize();
> > 	if (err)
> > 		return ret;
> >
> > 	err = ancillary_device_add();
> > 	if (ret)
> > 		goto err_unwind;
> >
> > 	err = some_foo();
> > 	if (err)
> > 		goto err_foo;
> > 	return 0;
> >
> > err_foo:
> > 	ancillary_device_del(adev);
> > err_unwind:
> > 	ancillary_device_put(adev->dev);
> > 	return err;
> > }
> >
> > cleanup()
> > {
> > 	ancillary_device_de(adev);
> > 	ancillary_device_put(adev);
> > 	/* It is common to have a one wrapper for this as
> > ancillary_device_unregister().
> > 	 * This will match with core device_unregister() that has precise
> > documentation.
> > 	 * but given fact that init() code need proper error unwinding, like
> > above,
> > 	 * it make sense to have two APIs, and no need to export another
> > symbol for unregister().
> > 	 * This pattern is very easy to audit and code.
> > 	 */
> > }
>
> I like this flow +1
>
> But ... since the init() function is performing both device_init and
> device_add - it should probably be called ancillary_device_register,
> and we are back to a single exported API for both register and
> unregister.
>
> At that point, do we need wrappers on the primitives init, add, del,
> and put?

Let me summarize.
1. You are not providing driver/core API but simplification and obfuscation
of basic primitives and structures. This is new layer. There is no room for
a claim that we must to follow internal API.
2. API should be symmetric. If you call to _register()/_add(), you will need
to call to _unregister()/_del(). Please don't add obscure _put().
3. You can't "ask" from users to call internal calls (put_device) over internal
fields in ancillary_device.
4. This API should be clear to drivers authors, "device_add()" call (and
semantic) is not used by the drivers (git grep " device_add(" drivers/).

Thanks

>
> -DaveE