Re: [PATCH V7 1/4] Documentation/devicetree/bindings: b850v3_lvds_dp

[Peter Senna Tschudin]

On Thu, Jan 19, 2017 at 10:17:45AM +0200, Laurent Pinchart wrote:
> Hi Peter,
> 
> On Thursday 19 Jan 2017 09:12:14 Peter Senna Tschudin wrote:
> > On Wed, Jan 18, 2017 at 11:10:58PM +0200, Laurent Pinchart wrote:
> > > On Monday 16 Jan 2017 09:37:11 Peter Senna Tschudin wrote:
> > >> On Tue, Jan 10, 2017 at 11:04:58PM +0200, Laurent Pinchart wrote:
> > >>> On Saturday 07 Jan 2017 01:29:52 Peter Senna Tschudin wrote:
> > >>>> On 04 January, 2017 21:39 CET, Rob Herring wrote:
> > >>>>> On Tue, Jan 3, 2017 at 5:34 PM, Peter Senna Tschudin wrote:
> > >>>>>> On 03 January, 2017 23:51 CET, Rob Herring <robh@xxxxxxxxxx> wrote:
> > >>>>>>> On Sun, Jan 01, 2017 at 09:24:29PM +0100, Peter Senna Tschudin 
> wrote:
> > >>>>>>>> Devicetree bindings documentation for the GE B850v3 LVDS/DP++
> > >>>>>>>> display bridge.
> > >>>>>>>> 
> > >>>>>>>> Cc: Martyn Welch <martyn.welch@xxxxxxxxxxxxxxx>
> > >>>>>>>> Cc: Martin Donnelly <martin.donnelly@xxxxxx>
> > >>>>>>>> Cc: Javier Martinez Canillas <javier@xxxxxxxxxxxx>
> > >>>>>>>> Cc: Enric Balletbo i Serra <enric.balletbo@xxxxxxxxxxxxx>
> > >>>>>>>> Cc: Philipp Zabel <p.zabel@xxxxxxxxxxxxxx>
> > >>>>>>>> Cc: Rob Herring <robh@xxxxxxxxxx>
> > >>>>>>>> Cc: Fabio Estevam <fabio.estevam@xxxxxxx>
> > >>>>>>>> Signed-off-by: Peter Senna Tschudin <peter.senna@xxxxxxxxxxxxx>
> > >>>>>>>> ---
> > >>>>>>>> There was an Acked-by from Rob Herring <robh@xxxxxxxxxx> for V6,
> > >>>>>>>> but I changed the bindings to use i2c_new_secondary_device() so I
> > >>>>>>>> removed it from the commit message.
> > 
> > [...]
> > 
> > >>>>>>>>  .../devicetree/bindings/ge/b850v3-lvds-dp.txt      | 39 +++++++++
> > >>>>>>> 
> > >>>>>>> Isn't '-lvds-dp' redundant? The part# should be enough.
> > >>>>>> 
> > >>>>>> b850v3 is the name of the product, this is why the proposed name.
> > >>>>>> What about, b850v3-dp2 dp2 indicating the second DP output?
> > >>>>> 
> > >>>>> Humm, b850v3 is the board name? This node should be the name of the
> > >>>>> bridge chip.
> > >>>> 
> > >>>> From the cover letter:
> > >>>> 
> > >>>> -- // --
> > >>>> There are two physical bridges on the video signal pipeline: a
> > >>>> STDP4028(LVDS to DP) and a STDP2690(DP to DP++).  The hardware and
> > >>>> firmware made it complicated for this binding to comprise two device
> > >>>> tree nodes, as the design goal is to configure both bridges based on
> > >>>> the LVDS signal, which leave the driver powerless to control the
> > >>>> video processing pipeline. The two bridges behaves as a single bridge,
> > >>>> and the driver is only needed for telling the host about EDID / HPD,
> > >>>> and for giving the host powers to ack interrupts. The video signal
> > >>>> pipeline
> > >>>> 
> > >>>> is as follows:
> > >>>>   Host -> LVDS|--(STDP4028)--|DP -> DP|--(STDP2690)--|DP++ -> Video
> > >>>>   output
> > >>>> 
> > >>>> -- // --
> > >>> 
> > >>> You forgot to prefix your patch series with [HACK] ;-)
> > >>> 
> > >>> How about fixing the issues that make the two DT nodes solution
> > >>> difficult ? What are they ?
> > >> 
> > >> The Firmware and the hardware design. Both bridges, with stock firmware,
> > >> are fully capable of providig EDID information and handling interrupts.
> > >> But on this specific design, with this specific firmware, I need to read
> > >> EDID from one bridge, and handle interrupts on the other.
> > > 
> > > Which firmware are you talking about ? Firmware running on the bridges, or
> > > somewhere else ?
> > 
> > Each bridge has it's own external flash containing a binary firmware.
> > The goal of the firmware is to configure the output end based on the
> > input end. This is part of what makes handling EDID and HPD challenging.
> > 
> > >> Back when I was starting the development I could not come up with a
> > >> proper way to split EDID and interrupts between two bridges in a way
> > >> that would result in a fully functional connector. Did I miss something?
> > > 
> > > You didn't, we did :-) I've been telling for quite some time now that we
> > > must decouple bridges from connectors, and this is another example of why
> > > we have such a need. Bridges should expose additional functions needed to
> > > implement connector operations, and the connector should be instantiated
> > > by the display driver with the help of bridge operations. You could then
> > > create a connector that relies on one bridge to read the EDID and on the
> > > other bridge to handle HPD.
> > 
> > Ah thanks. So for now the single DT node approach is acceptable, right?
> > The problem is that even if the driver is getting better on each
> > iteration, the single DT node for two chips issue comes back often and I
> > believe is _the_ issue preventing the driver from getting upstream. V1
> > was sent ~ 8 months ago...
> > 
> > Can I have some blessing on the single DT node approach for now?
> 
> With the "DT as an ABI" approach, I'm afraid not. Temporary hacks are 
> acceptable on the driver side, but you need two nodes in DT.

So can I make two node DT "in the right way" and work around current
connectors vs. bridge limitations on the driver side? This seems to be
doable.

Then I could fix bridge API, with my own driver and update API clients
affected by the change...

> 
> > I'm one of the 3 proposed maintainers for the driver, and I'm willing to
> > maintain the driver on the long run, as is the same with the other two
> > proposed maintainers. So when the time to split the node in two comes,
> > we will be around, and willing to do it ourselves.
> 
> How about putting that team of 3 maintainers to work on fixing the problem in 
> the bridge API ? :-)

Guess you would be a good lawyer! My point was not exactly that we could
work in parallel. Point was that there is redundancy in case one or two
of us loose interest. But nice try! :-)

Chances of having resources to fix bridge API and clients were better 6
months ago, but let me see what I can get.  Last blocking issue was the
migration to atomic, now this. I'm going to need to answer what the next
blocking issue is going to be.

Actually in these ~8 months one bit of the required changes was
accepted: dc80d7038883, but this was generic and not related to our
specific use case.

Thanks!

Peter

> 
> -- 
> Regards,
> 
> Laurent Pinchart
>