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@xxxxxxxxx> 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@xxxxxxxxxx>
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