Re: [PATCH v2] sound/hda: Add NULL check to component match callback function

[Takashi Iwai <tiwai@suse.de>]

On Thu, 31 Mar 2022 17:33:03 +0200,
Benson Leung wrote:
> 
> Hi Takashi,
> 
> On Thu, Mar 31, 2022 at 04:19:15PM +0200, Takashi Iwai wrote:
> > On Thu, 31 Mar 2022 15:29:10 +0200,
> > Takashi Iwai wrote:
> > > 
> > > On Thu, 31 Mar 2022 11:45:47 +0200,
> > > Takashi Iwai wrote:
> > > > 
> > > > On Thu, 31 Mar 2022 11:34:38 +0200,
> > > > Heikki Krogerus wrote:
> > > > > 
> > > > > On Thu, Mar 31, 2022 at 11:28:20AM +0200, Takashi Iwai wrote:
> > > > > > On Thu, 31 Mar 2022 11:25:43 +0200,
> > > > > > Heikki Krogerus wrote:
> > > > > > > 
> > > > > > > On Thu, Mar 31, 2022 at 11:12:55AM +0200, Takashi Iwai wrote:
> > > > > > > > > > > -     if (!strcmp(dev->driver->name, "i915") &&
> > > > > > > > > > > +     if (dev->driver && !strcmp(dev->driver->name, "i915") &&
> > > > > > > > > >
> > > > > > > > > > Can NULL dev->driver be really seen?  I thought the components are
> > > > > > > > > > added by the drivers, hence they ought to have the driver field set.
> > > > > > > > > > But there can be corner cases I overlooked.
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > thanks,
> > > > > > > > > >
> > > > > > > > > > Takashi
> > > > > > > > > 
> > > > > > > > > Hi Takashi,
> > > > > > > > > 
> > > > > > > > > When I try using component_add in a different driver (usb4 in my
> > > > > > > > > case), I think dev->driver here is NULL because the i915 drivers do
> > > > > > > > > not have their component master fully bound when this new component is
> > > > > > > > > registered. When I test it, it seems to be causing a crash.
> > > > > > > > 
> > > > > > > > Hm, from where component_add*() is called?  Basically dev->driver must
> > > > > > > > be already set before the corresponding driver gets bound at
> > > > > > > > __driver_probe_deviec().  So, if the device is added to component from
> > > > > > > > the corresponding driver's probe, dev->driver must be non-NULL.
> > > > > > > 
> > > > > > > The code that declares a device as component does not have to be the
> > > > > > > driver of that device.
> > > > > > > 
> > > > > > > In our case the components are USB ports, and they are devices that
> > > > > > > are actually never bind to any drivers: drivers/usb/core/port.c
> > > > > > 
> > > > > > OK, that's what I wanted to know.  It'd be helpful if it's more
> > > > > > clearly mentioned in the commit log.
> > > > > 
> > > > > Agree.
> > > > > 
> > > > > > BTW, the same problem must be seen in MEI drivers, too.
> > > > > 
> > > > > Wasn't there a patch for those too? I lost track...
> > > > 
> > > > I don't know, I just checked the latest Linus tree.
> > > > 
> > > > And, looking at the HD-audio code, I still wonder how NULL dev->driver
> > > > can reach there.  Is there any PCI device that is added to component
> > > > without binding to a driver?  We have dev_is_pci() check at the
> > > > beginning, so non-PCI devices should bail out there...
> > > 
> > > Further reading on, I'm really confused.  How data=NULL can be passed
> > > to this function?  The data argument is the value passed from the
> > > component_match_add_typed() call in HD-audio driver, hence it must be
> > > always the snd_hdac_bus object.
> > > 
> > > And, I guess the i915 string check can be omitted completely, at
> > > least, for HD-audio driver.  It already have a check of the parent of
> > > the device and that should be enough.
> > 
> > That said, something like below (supposing data NULL check being
> > superfluous), instead.
> > 
> > 
> > Takashi
> > 
> > --- a/sound/hda/hdac_i915.c
> > +++ b/sound/hda/hdac_i915.c
> > @@ -102,18 +102,13 @@ static int i915_component_master_match(struct device *dev, int subcomponent,
> >  	struct pci_dev *hdac_pci, *i915_pci;
> >  	struct hdac_bus *bus = data;
> >  
> > -	if (!dev_is_pci(dev))
> > +	if (subcomponent != I915_COMPONENT_AUDIO || !dev_is_pci(dev))
> >  		return 0;
> >  
> 
> If I recall this bug correctly, it's not the usb port perse that is falling
> through this !dev_is_pci(dev) check, it's actually the usb4-port in a new
> proposed patch by Heikki and Mika to extend the usb type-c component to
> encompass the usb4 specific pieces too. Is it possible usb4 ports are considered
> pci devices, and that's how we got into this situation?

Yes, that explains for one of two changes in the original patch.
But why data==NULL check is needed is still unclear.

> Also, a little more background information: This crash happens because in
> our kernel configs, we config'd the usb4 driver as =y (built in) instead of
> =m module, which meant that the usb4 port's driver was adding a component
> likely much earlier than hdac_i915.

Thanks, it's what I supposed, too.


Takashi