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

[Thierry Reding <thierry.reding@gmail.com>]

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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@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@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"

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.

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

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