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

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Date:   Fri, 22 May 2020 16:44:18 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
Cc:     Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        davem@...emloft.net, gregkh@...uxfoundation.org,
        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 Fri, May 22, 2020 at 01:48:00PM -0500, Pierre-Louis Bossart wrote:
> 
> 
> On 5/22/20 1:40 PM, Jason Gunthorpe wrote:
> > On Fri, May 22, 2020 at 01:35:54PM -0500, Pierre-Louis Bossart wrote:
> > > 
> > > 
> > > On 5/22/20 12:10 PM, Jason Gunthorpe wrote:
> > > > On Fri, May 22, 2020 at 10:33:20AM -0500, Pierre-Louis Bossart wrote:
> > > > 
> > > > > > Maybe not great, but at least it is consistent with all the lifetime
> > > > > > models and the operation of the driver core.
> > > > > 
> > > > > I agree your comments are valid ones, I just don't have a solution to be
> > > > > fully compliant with these models and report failures of the driver probe
> > > > > for a child device due to configuration issues (bad audio topology, etc).
> > > > 
> > > > 
> > > > > My understanding is that errors on probe are explicitly not handled in the
> > > > > driver core, see e.g. comments such as:
> > > > 
> > > > Yes, but that doesn't really apply here...
> > > > > /*
> > > > >    * Ignore errors returned by ->probe so that the next driver can try
> > > > >    * its luck.
> > > > >    */
> > > > > https://elixir.bootlin.com/linux/latest/source/drivers/base/dd.c#L636
> > > > > 
> > > > > If somehow we could request the error to be reported then probably we
> > > > > wouldn't need this complete/wait_for_completion mechanism as a custom
> > > > > notification.
> > > > 
> > > > That is the same issue as the completion, a driver should not be
> > > > making assumptions about ordering like this. For instance what if the
> > > > current driver is in the initrd and the 2nd driver is in a module in
> > > > the filesystem? It will not probe until the system boots more
> > > > completely.
> > > > 
> > > > This is all stuff that is supposed to work properly.
> > > > 
> > > > > Not at the moment, no. there are no failures reported in dmesg, and
> > > > > the user does not see any card created. This is a silent error.
> > > > 
> > > > Creating a partial non-function card until all the parts are loaded
> > > > seems like the right way to surface an error like this.
> > > > 
> > > > Or don't break the driver up in this manner if all the parts are really
> > > > required just for it to function - quite strange place to get into.
> > > 
> > > This is not about having all the parts available - that's handled already
> > > with deferred probe - but an error happening during card registration. In
> > > that case the ALSA/ASoC core throws an error and we cannot report it back to
> > > the parent.
> > 
> > The whole point of the virtual bus stuff was to split up a
> > multi-functional PCI device into parts. If all the parts are required
> > to be working to make the device work, why are you using virtual bus
> > here?
> 
> It's the other way around: how does the core know that one part isn't
> functional.

> There is nothing in what we said that requires that all parts are fully
> functional. All we stated is that when *one* part isn't fully functional we
> know about it.

Maybe if you can present some diagram or something, because I really
can't understand why asoc is trying to do with virtual bus here.

> > > > What happens if the user unplugs this sub driver once things start
> > > > running?
> > > 
> > > refcounting in the ALSA core prevents that from happening usually.
> > 
> > So user triggered unplug of driver that attaches here just hangs
> > forever? That isn't OK either.
> 
> No, you'd get a 'module in use' error if I am not mistaken.

You can disconnect drivers without unloading modules. It is a common
misconception. You should never, ever, rely on module ref counting for
anything more than keeping function pointers in memory.

Jason