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

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Message-ID: <YkVzl4NEzwDAp/Zq@kuha.fi.intel.com>
Date:   Thu, 31 Mar 2022 12:25:43 +0300
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     Won Chung <wonchung@...gle.com>, Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Benson Leung <bleung@...omium.org>,
        Prashant Malani <pmalani@...omium.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] sound/hda: Add NULL check to component match callback
 function

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

thanks,

-- 
heikki