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

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Date:   Fri, 9 Oct 2020 14:40:47 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
Cc:     Parav Pandit <parav@...dia.com>,
        "Ertman, David M" <david.m.ertman@...el.com>,
        "alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "tiwai@...e.de" <tiwai@...e.de>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ranjani.sridharan@...ux.intel.com" 
        <ranjani.sridharan@...ux.intel.com>,
        "fred.oh@...ux.intel.com" <fred.oh@...ux.intel.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "dledford@...hat.com" <dledford@...hat.com>,
        "broonie@...nel.org" <broonie@...nel.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "Saleem, Shiraz" <shiraz.saleem@...el.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "Patil, Kiran" <kiran.patil@...el.com>
Subject: Re: [PATCH v2 1/6] Add ancillary bus support

On Thu, Oct 08, 2020 at 08:29:00AM -0500, Pierre-Louis Bossart wrote:
>
> > > > > > 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.
> > > > >
> > > > > Kind reminder that we introduced the two functions to allow the
> > > > > caller to know if it needed to free memory when initialize() fails,
> > > > > and it didn't need to free memory when add() failed since
> > > > > put_device() takes care of it. If you have a single init() function
> > > > > it's impossible to know which behavior to select on error.
> > > > >
> > > > > I also have a case with SoundWire where it's nice to first
> > > > > initialize, then set some data and then add.
> > > > >
> > > >
> > > > The flow as outlined by Parav above does an initialize as the first
> > > > step, so every error path out of the function has to do a
> > > > put_device(), so you would never need to manually free the memory in
> > > the setup function.
> > > > It would be freed in the release call.
> > >
> > > err = ancillary_device_initialize();
> > > if (err)
> > > 	return ret;
> > >
> > > where is the put_device() here? if the release function does any sort of
> > > kfree, then you'd need to do it manually in this case.
> > Since device_initialize() failed, put_device() cannot be done here.
> > So yes, pseudo code should have shown,
> > if (err) {
> > 	kfree(adev);
> > 	return err;
> > }
>
> This doesn't work if the adev is part of a larger structure allocated by the
> parent, which is pretty much the intent to extent the basic bus and pass
> additional information which can be accessed with container_of().

Please take a look how ib_alloc_device() is implemented. It does all
that you wrote above in very similar manner to netdev_alloc.

In a nutshell, ib_alloc_device receives needed size from the user and
requires from the users to extend their structures below "general" one.

Thanks