linux-kernel - Re: [PATCH 1/2] module: add a function to add module references

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Date:   Fri, 29 Apr 2022 12:10:07 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Mauro Carvalho Chehab <mchehab@...nel.org>
Cc:     Daniel Vetter <daniel@...ll.ch>,
        Luis Chamberlain <mcgrof@...nel.org>, mauro.chehab@...el.com,
        Kai Vehmanen <kai.vehmanen@...el.com>,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        Pierre-Louis Bossart <pierre-louis.bossart@...el.com>,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-modules@...r.kernel.org,
        David Airlie <airlied@...ux.ie>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 1/2] module: add a function to add module references

On Fri, Apr 29, 2022 at 10:15:03AM +0100, Mauro Carvalho Chehab wrote:
> HI Greg,
> 
> Em Fri, 29 Apr 2022 10:30:33 +0200
> Greg KH <gregkh@...uxfoundation.org> escreveu:
> 
> > On Fri, Apr 29, 2022 at 09:07:57AM +0100, Mauro Carvalho Chehab wrote:
> > > Hi Daniel,
> > > 
> > > Em Fri, 29 Apr 2022 09:54:10 +0200
> > > Daniel Vetter <daniel@...ll.ch> escreveu:
> > >   
> > > > On Fri, Apr 29, 2022 at 07:31:15AM +0100, Mauro Carvalho Chehab wrote:  
> > > > > Sometimes, device drivers are bound using indirect references,
> > > > > which is not visible when looking at /proc/modules or lsmod.
> > > > > 
> > > > > Add a function to allow setting up module references for such
> > > > > cases.
> > > > > 
> > > > > Reviewed-by: Dan Williams <dan.j.williams@...el.com>
> > > > > Signed-off-by: Mauro Carvalho Chehab <mchehab@...nel.org>    
> > > > 
> > > > This sounds like duct tape at the wrong level. We should have a
> > > > device_link connecting these devices, and maybe device_link internally
> > > > needs to make sure the respective driver modules stay around for long
> > > > enough too. But open-coding this all over the place into every driver that
> > > > has some kind of cross-driver dependency sounds terrible.
> > > > 
> > > > Or maybe the bug is that the snd driver keeps accessing the hw/component
> > > > side when that is just plain gone. Iirc there's still fundamental issues
> > > > there on the sound side of things, which have been attempted to paper over
> > > > by timeouts and stuff like that in the past instead of enforcing a hard
> > > > link between the snd and i915 side.  
> > > 
> > > I agree with you that the device link between snd-hda and the DRM driver
> > > should properly handle unbinding on both directions. This is something
> > > that require further discussions with ALSA and DRM people, and we should
> > > keep working on it.
> > > 
> > > Yet, the binding between those drivers do exist, but, despite other
> > > similar inter-driver bindings being properly reported by lsmod, this one
> > > is invisible for userspace.
> > > 
> > > What this series does is to make such binding visible. As simple as that.  
> > 
> > It also increases the reference count, and creates a user/kernel api
> > with the symlinks, right?  Will the reference count increase prevent the
> > modules from now being unloadable?
> >
> > This feels like a very "weak" link between modules that should not be
> > needed if reference counting is implemented properly (so that things are
> > cleaned up in the correct order.)
> 
> The refcount increment exists even without this patch, as
> hda_component_master_bind() at sound/hda/hdac_component.c uses 
> try_module_get() when it creates the device link.

Ok, then why shouldn't try_module_get() be creating this link instead of
having to manually do it this way again?  You don't want to have to go
around and add this call to all users of that function, right?

thanks,

greg k-h