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

[Rob Clark]

On Fri, Mar 26, 2021 at 5:38 AM Thierry Reding <thierry.reding@xxxxxxxxx> wrote:
>
> On Wed, Mar 17, 2021 at 06:53:04PM -0700, Rob Clark wrote:
> > On Wed, Mar 17, 2021 at 4:27 PM Matthias Kaehlcke <mka@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > > > ---
> > >
> > > 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"
>
> This is practically no different than the "white lie". I suppose you
> could perhaps call it "more honest", if you want.
>
> The point remains that unless we describe exactly which panel we're
> dealing with, we ultimately have no way of properly quirking anything if
> we ever have to. Also, once we allow this kind of wildcard we can
> suddenly get into a situation where people might want to reuse this on
> something that's not at all a google-pompom board because the same
> particular power sequence happens to work on on some other board.
>
> Similarly I can imagine a situation where we could now have the same
> panel supported by multiple different wildcard compatible strings. How
> is that supposed to be any cleaner than what we have now?
>
> And I still keep wondering why bootloaders can't be taught about these
> kinds of things. We have in the past discussed various solutions where
> the bootloader could detect the type of panel connected and set the
> proper compatible string.

The bootloader cannot detect the panel without powering up the panel,
which it normally does not do if you are not in dev-mode (it would add
a significant amount of time to bootup, which is why we can't do this)

BR,
-R

> If that's too complicated and these really are standardized interfaces
> that work across a wide range of devices with perhaps a couple of
> standard parameter, then introducing a standard connector type like
> Rob Herring is suggesting makes more sense because that more properly
> describes where exactly the standardization is going on (i.e. at the
> interface level rather than the panel level).
>
> Thierry