netdev - Re: [net-next v4 10/12] ASoC: SOF: Introduce descriptors for SOF client

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Date:   Wed, 27 May 2020 09:17:33 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        davem@...emloft.net, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, nhorman@...hat.com,
        sassmann@...hat.com, Fred Oh <fred.oh@...ux.intel.com>
Subject: Re: [net-next v4 10/12] ASoC: SOF: Introduce descriptors for SOF
 client

On Tue, May 26, 2020 at 03:37:36PM +0200, Takashi Iwai wrote:
> On Tue, 26 May 2020 15:15:26 +0200,
> Pierre-Louis Bossart wrote:
> > 
> > 
> > 
> > On 5/24/20 1:35 AM, Greg KH wrote:
> > > On Sat, May 23, 2020 at 02:41:51PM -0500, Pierre-Louis Bossart wrote:
> > >>
> > >>
> > >> On 5/23/20 1:23 AM, Greg KH wrote:
> > >>> On Fri, May 22, 2020 at 09:29:57AM -0500, Pierre-Louis Bossart wrote:
> > >>>> This is not an hypothetical case, we've had this recurring problem when a
> > >>>> PCI device creates an audio card represented as a platform device. When the
> > >>>> card registration fails, typically due to configuration issues, the PCI
> > >>>> probe still completes.
> > >>>
> > >>> Then fix that problem there.  The audio card should not be being created
> > >>> as a platform device, as that is not what it is.  And even if it was,
> > >>> the probe should not complete, it should clean up after itself and error
> > >>> out.
> > >>
> > >> Did you mean 'the PCI probe should not complete and error out'?
> > >
> > > Yes.
> > >
> > >> If yes, that's yet another problem... During the PCI probe, we start a
> > >> workqueue and return success to avoid blocking everything.
> > >
> > > That's crazy.
> > >
> > >> And only 'later' do we actually create the card. So that's two levels
> > >> of probe that cannot report a failure. I didn't come up with this
> > >> design, IIRC this is due to audio-DRM dependencies and it's been used
> > >> for 10+ years.
> > >
> > > Then if the probe function fails, it needs to unwind everything itself
> > > and unregister the device with the PCI subsystem so that things work
> > > properly.  If it does not do that today, that's a bug.
> > >
> > > What kind of crazy dependencies cause this type of "requirement"?
> > 
> > I think it is related to the request_module("i915") in
> > snd_hdac_i915_init(), and possibly other firmware download.
> > 
> > Adding Takashi for more details.
> 
> Right, there are a few levels of complexity there.  The HD-audio
> PCI controller driver, for example, is initialized in an async way
> with a work.  It loads the firmware files with
> request_firmware_nowait() and also binds itself as a component master
> with the DRM graphics driver via component framework.
> 
> Currently it has no way to unwind the PCI binding itself at the error
> path, though.  In theory it should be possible to unregister the PCI
> from the driver itself in the work context, but it failed in the
> earlier experiments, hence the driver sets itself in a disabled state
> instead.  Maybe worth to try again.
> 
> But, to be noted, all belonging sub-devices aren't instantiated but
> deleted at the error path.  Only the main PCI binding is kept in a
> disabled state just as a place holder until it's unbound explicitly.

Ok, that's good to hear.  But platform devices should never be showing
up as a child of a PCI device.  In the "near future" when we get the
virtual bus code merged, we can convert any existing users like this to
the new code.

thanks,

greg k-h