Re: [PATCH v3 17/25] dt-bindings: panel: Add Bananapi S070WV20-CT16 ICN6211 MIPI-DSI to RGB bridge

[Jagan Teki]

On Wed, Nov 7, 2018 at 2:41 PM Andrzej Hajda <a.hajda@xxxxxxxxxxx> wrote:
>
> On 06.11.2018 19:08, Jagan Teki wrote:
> > On Wed, Oct 31, 2018 at 2:45 PM Andrzej Hajda <a.hajda@xxxxxxxxxxx> wrote:
> >> On 31.10.2018 09:58, Chen-Yu Tsai wrote:
> >>> On Wed, Oct 31, 2018 at 4:53 PM Andrzej Hajda <a.hajda@xxxxxxxxxxx> wrote:
> >>>> On 26.10.2018 16:43, Jagan Teki wrote:
> >>>>> Bananapi S070WV20-CT16 ICN6211 is 800x480, 4-lane MIPI-DSI to RGB
> >>>>> bridge panel, which is available on same PCB with 24-bit RGB interface.
> >>>>>
> >>>>> So, this patch adds DSI specific binding details on existing
> >>>>> dt-bindings file.
> >>>>>
> >>>>> Signed-off-by: Jagan Teki <jagan@xxxxxxxxxxxxxxxxxxxx>
> >>>>> ---
> >>>>> Changes for v3:
> >>>>> - Use existing binding doc and update dsi details
> >>>>> Changes for v2:
> >>>>> - none
> >>>>>
> >>>>>  .../display/panel/bananapi,s070wv20-ct16.txt  | 31 +++++++++++++++++--
> >>>>>  1 file changed, 29 insertions(+), 2 deletions(-)
> >>>>>
> >>>>> diff --git a/Documentation/devicetree/bindings/display/panel/bananapi,s070wv20-ct16.txt b/Documentation/devicetree/bindings/display/panel/bananapi,s070wv20-ct16.txt
> >>>>> index 35bc0c839f49..b7855dc7c66f 100644
> >>>>> --- a/Documentation/devicetree/bindings/display/panel/bananapi,s070wv20-ct16.txt
> >>>>> +++ b/Documentation/devicetree/bindings/display/panel/bananapi,s070wv20-ct16.txt
> >>>>> @@ -1,12 +1,39 @@
> >>>>>  Banana Pi 7" (S070WV20-CT16) TFT LCD Panel
> >>>>>
> >>>>> +S070WV20-CT16 is 7" 800x480 panel connected through a 24-bit RGB interface.
> >>>>> +
> >>>>> +Depending on the variant, the PCB attached to the panel module either
> >>>>> +supports DSI, or DSI + 24-bit RGB. DSI is converted to 24-bit RGB via
> >>>>> +an onboard ICN6211 MIPI DSI - RGB bridge chip, then fed to the panel
> >>>>> +itself
> >>>> As I understand this is display board, which contains 'pure' RGB panel
> >>>> S070WV20-CT16 and optionally ICN6211 DSI->RGB bridge.
> >>>> These are separate devices, just connected by vendor to simplify its
> >>>> assembly. Why don't you create then bridge driver for ICN6211 and RGB
> >>>> panel driver for S070WV20-CT16 - it looks more generic.
> >>>> Then you can describe both in dts and voila.
> >>>> Creating drivers for every combo of devices (panel + bridge), just
> >>>> because some vendor sells them together seems incorrect - we have
> >>>> devicetree for it.
> >>> Rob suggested this, and also the opposite: using the same
> >>> "bananapi,s070wv20-ct16"
> >>> compatible string for both types of connections, and have the driver deal with
> >>> detecting the bus type.
> >>>
> >>> The thing about the bridge chip is that there's no available datasheet that
> >>> describes all the parts of the init sequence, in fact none at all. I managed
> >>> to work out some bits, but the others remain a mystery and must be hard-coded
> >>> to match the panel. That would work against having a generic bridge driver.
> >>
> >> But it is common for many chips - 1st version of the driver is developed
> >> on one platform and it supports only one configuration, if next platform
> >> with the same cheap appears the driver is augmented if necessary.
> > At-least few of the commands from panel initialization code, the
> > respective opcode data values are based on panel timings and even
> > clock value is different in DSI. I think it look hard to try bridge
> > driver for these restrictions, do you have any suggestions?
>
>
> Where do you see an issue? Since panel is RGB it should have no
> initialization sequence (beside regulator/gpio power on/off), so the
> only thing to do is to figure out which regulators/gpios belongs to
> which component - with publicly available specs it should be doable.
>
> The whole initialization sequence is for the bridge, so you put it into
> bridge driver, for starters it can be hardcoded.

Yes, I understand we can move regulators/gpio setup separately and
though we hardcode the init sequence there is difference  in clock for
DSI(which I mentioned in previous mail). DSI panel can't work with
clock used by RGB panel-simple.

>
> Then you can:
>
> 1. Try to find other users of this ICN6211 chip and compare
> initialization sequences to guess purpose of registers.
>
> 2. Try to get specs of the chip (ask vendor, distributor, grep Internet).

As we mentioned (even Chen-Yu), we are unable to find the proper spec
for this panel, all we taken reference from AW BSP code.

>
> 3. Do nothing - if there will be other users of the bridge they will do
> this work.

Don't know how we can go with generic bridge driver irrespective of
these particular wrinkles, let me know if you have any suggestions.