Nouveau - Apr 2021 - [PATCH v3 03/20] drm/dp: Move i2c init to drm_dp_aux_init, add __must_check and fini

On Thu, 2021-04-22 at 18:33 -0400, Lyude Paul wrote:
> OK - talked with Ville a bit on this and did some of my own research, I
> actually think that moving i2c to drm_dp_aux_init() is the right decision
> for
> the time being. The reasoning behind this being that as shown by my
previous
> work of fixing drivers that call drm_dp_aux_register() too early - it seems
> like there's already been drivers that have been working just fine with
> setting up the i2c device before DRM registration. 
> 
> In the future, it'd probably be better if we can split up
i2c_add_adapter()
> into an init and register function - but we'll have to talk with the
i2c
> maintainers to see if this is acceptable w/ them
Actually - I think adding the ability to refcount dp aux adapters might be a
better solution so I'm going to try that!
> 
> On Thu, 2021-04-22 at 13:18 -0400, Lyude Paul wrote:
> > On Tue, 2021-04-20 at 02:16 +0300, Ville Syrj?l? wrote:
> > > 
> > > The init vs. register split is intentional. Registering the thing
> > > and allowing userspace access to it before the rest of the driver
> > > is ready isn't particularly great. For a while now we've
tried to
> > > move towards an architecture where the driver is fully
initialzied
> > > before anything gets exposed to userspace.
> > 
> > Yeah-thank you for pointing this out. Thierry - do you think
there's an
> > alternate solution we could go with in Tegra to fix the get_device()
issue
> > that wouldn't require us trying to expose the i2c adapter early?
> > 
> > > 
> > 
> 
-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

On Fri, Apr 23, 2021 at 12:11:06AM -0400, Lyude Paul
wrote:
> On Thu, 2021-04-22 at 18:33 -0400, Lyude Paul wrote:
> > OK - talked with Ville a bit on this and did some of my own research,
I
> > actually think that moving i2c to drm_dp_aux_init() is the right
decision
> > for
> > the time being. The reasoning behind this being that as shown by my
previous
> > work of fixing drivers that call drm_dp_aux_register() too early - it
seems
> > like there's already been drivers that have been working just fine
with
> > setting up the i2c device before DRM registration. 
> > 
> > In the future, it'd probably be better if we can split up
i2c_add_adapter()
> > into an init and register function - but we'll have to talk with
the i2c
> > maintainers to see if this is acceptable w/ them
> 
> Actually - I think adding the ability to refcount dp aux adapters might be
a
> better solution so I'm going to try that!
I'm curious: how is a DP AUX adapter reference count going to solve the
issue of potentially registering devices too early (i.e. before the DRM
is registered)?

Is it because registering too early could cause a reference count
problem if somebody get a hold of the DP AUX adapter before the parent
DRM device is around?

Thierry
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