Re: [PATCH] drm: don't link DP aux i2c adapter to the hardware device node

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Nov 14, 2017 at 03:34:19PM +0100, Andrzej Hajda wrote:
> On 05.04.2017 14:04, Thierry Reding wrote:
> > On Wed, Apr 05, 2017 at 10:52:32AM +0200, Lucas Stach wrote:
> >> Hi Rob,
> >>
> >> Am Mittwoch, den 29.03.2017, 08:56 -0500 schrieb Rob Herring:
> >>> On Mon, Jan 23, 2017 at 10:33 AM, Thierry Reding
> >>> <thierry.reding@gmail.com> wrote:
> >>>> On Fri, Jan 13, 2017 at 06:36:30PM +0100, Lucas Stach wrote:
> >>>>> The i2c adapter on DP AUX is purely a software construct. Linking
> >>>>> it to the device node of the parent device is wrong, as it leads to
> >>>>> 2 devices sharing the same device node, which is bad practice, as
> >>>> Who says that two devices can't share the same device node? It's done
> >>>> all the time.
> >>> It's done *some of the time* and I would not consider it best practice.
> >>>
> >>>>> well as the i2c trying to populate children of the i2c adapter by
> >>>>> looking at the child device nodes of the parent device.
> >>>> A set of patches landed in v4.9 to work around this issue in a better
> >>>> way. See:
> >>>>
> >>>>         98b00488459e dt-bindings: i2c: Add support for 'i2c-bus' subnode
> >>>>         7e4c224abfe8 i2c: core: Add support for 'i2c-bus' subnode
> >>> What does this buy us? I don't see why this needs to be in DT either.
> >>> Contrary to popular belief, DT is not the only way to instantiate
> >>> devices, C code can still do it.
> >>>
> >>> Also, if this one line removal has no side effects, then how was it
> >>> even needed? We can always add it back if there's some argument for
> >>> why it is needed.
> >> Okay, so I take this as you mostly agreeing with the rationale of this
> >> patch.
> > For some general background on this: I was originally using this for DP
> > support on Tegra (though that ended up never getting merged because of a
> > particularily frustrating episode of trying to get better link training
> > support into the core helpers) and use it as a means to obtain the I2C
> > controller used for DDC. On Tegra, and I suspect other devices as well,
> > the DP AUX controller is separate from the encoder, so the idea was to
> > link them together using a standard ddc-i2c-bus phandle.
> >
> > I ended up not needing that because the encoder and DP AUX controller
> > are so tightly linked on Tegra that I need direct access to the DP AUX
> > anyway and can therefore directly get the I2C controller from that.
> >
> > If there aren't any other users of this, I suppose we could simply
> > remove the line. Should someone turn up in the future and require the
> > I2C controller to be looked up from a phandle we could add it again,
> > at which point we'd have to investigate again how to get rid of the
> > errors.
> >
> > Acked-by: Thierry Reding <treding@nvidia.com>
> 
> It looks like the patch has been accepted but not merged. Who can pick
> it up? or is it OK if I put it int drm-misc?

I don't have the patch itself, but sounds good. Just go ahead and push.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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