[RFC PATCH 1/3] dt-bindings: display: simple: Add the panel on sc7180-trogdor-pompom

[Rob Clark]

On Wed, Mar 17, 2021 at 4:27 PM Matthias Kaehlcke <mka at chromium.org> wrote:
>
> On Tue, Mar 16, 2021 at 02:08:19PM -0700, Douglas Anderson wrote:
> > The sc7180-trogdor-pompom board might be attached to any number of a
> > pile of eDP panels. At the moment I'm told that the list might include:
> > - KD KD116N21-30NV-A010
> > - KD KD116N09-30NH-A016
> > - Starry 2081116HHD028001-51D
> > - Sharp LQ116M1JW10
> >
> > It should be noted that while the EDID programmed in the first 3
> > panels indicates that they should run with exactly the same timing (to
> > keep things simple), the 4th panel not only needs different timing but
> > has a different resolution.
> >
> > As is true in general with eDP panels, we can figure out which panel
> > we have and all the info needed to drive its pixel clock by reading
> > the EDID. However, we can do this only after we've powered the panel
> > on. Powering on the panels requires following the timing diagram in
> > each panel's datasheet which specifies delays between certain
> > actions. This means that, while we can be quite dynamic about handling
> > things we can't just totally skip out on describing the panel like we
> > could do if it was connected to an external-facing DP port.
> >
> > While the different panels have slightly different delays, it's
> > possible to come up with a set of unified delays that will work on all
> > the panels. From reading the datasheets:
> > * KD KD116N21-30NV-A010 and KD KD116N09-30NH-A016
> >   - HPD absent delay: 200 ms
> >   - Unprepare delay: 150 ms (datasheet is confusing, might be 500 ms)
> > * Starry 2081116HHD028001-51D
> >   - HPD absent delay: 100 ms
> >   - Enable delay: (link training done till enable BL): 200 ms
> >   - Unprepare delay: 500 ms
> > * Sharp LQ116M1JW10
> >   - HPD absent delay: 200 ms
> >   - Unprepare delay: 500 ms
> >   - Prepare to enable delay (power on till backlight): 100 ms
> >
> > Unified:
> > - HPD absent delay: 200 ms
> > - Unprepare delay: 500 ms
> > - Enable delay: 200 ms
> >
> > NOTE: in theory the only thing that we _really_ need unity on is the
> > "HPD absent delay" since once the panel asserts HPD we can read the
> > EDID and could make per-panel decisions if we wanted.
> >
> > Let's create a definition of "a panel that can be attached to pompom"
> > as a panel that provides a valid EDID and can work with the standard
> > pompom power sequencing. If more panels are later attached to pompom
> > then it's fine as long as they work in a compatible way.
> >
> > One might ask why we can't just use a generic string here and provide
> > the timings directly in the device tree file. As I understand it,
> > trying to describe generic power sequencing in the device tree is
> > frowned upon and the one instance (SD/MMC) is regarded as a mistake
> > that shouldn't be repeated. Specifying a power sequence per board (or
> > per board class) feels like a reasonable compromise. We're not trying
> > to define fully generic power sequence bindings but we can also take
> > advantage of the semi-probable properties of the attached device.
> >
> > NOTE: I believe that past instances of supporting this type of thing
> > have used the "white lie" approach. One representative panel was
> > listed in the device tree. The power sequencings of this
> > representative panel were OK to use across all panels that might be
> > attached and other differences were handled by EDID. This patch
> > attempts to set a new precedent and avoid the need for the white lie.
> >
> > Signed-off-by: Douglas Anderson <dianders at chromium.org>
> > ---
>
> Sounds reasonable to me if DT maintainers can live with this abstract
> hardware definition. It's clearer than the 'white lie' approach.

Yeah, it is a weird grey area between "discoverable" and "not
discoverable".. but I favor DT reflecting reality as much as
possible/feasible, so I think this is definity cleaner than "white
lies"

Reviewed-by: Rob Clark <robdclark at gmail.com>

> It's then up to the vendor/manufacturer to ensure to only ship devices
> with panels that have compatible timings.
>
> >  .../devicetree/bindings/display/panel/panel-simple.yaml       | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/Documentation/devicetree/bindings/display/panel/panel-simple.yaml b/Documentation/devicetree/bindings/display/panel/panel-simple.yaml
> > index 62b0d54d87b7..9807dbc1cceb 100644
> > --- a/Documentation/devicetree/bindings/display/panel/panel-simple.yaml
> > +++ b/Documentation/devicetree/bindings/display/panel/panel-simple.yaml
> > @@ -140,6 +140,10 @@ properties:
> >        - giantplus,gpg48273qs5
> >          # GiantPlus GPM940B0 3.0" QVGA TFT LCD panel
> >        - giantplus,gpm940b0
> > +        # A panel connected to a google,pompom board. Panel is guaranteed to
> > +        # confirm to google,pompom-panel power sequencing requirements and then
>
> s/confirm/conform/ ?
>
> > +        # the specific panel will be probed via EDID.
> > +      - google,pompom-panel
> >          # HannStar Display Corp. HSD070PWW1 7.0" WXGA TFT LCD panel
> >        - hannstar,hsd070pww1
> >          # HannStar Display Corp. HSD100PXN1 10.1" XGA LVDS panel
>
> FWIW:
>
> Reviewed-by: Matthias Kaehlcke <mka at chromium.org>
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