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

[Rob Clark]

On Fri, Mar 26, 2021 at 4:48 PM Rob Herring <robh+dt@xxxxxxxxxx> wrote:
>
> On Fri, Mar 26, 2021 at 4:13 PM Rob Clark <robdclark@xxxxxxxxx> wrote:
> >
> > On Fri, Mar 26, 2021 at 12:48 PM Rob Herring <robh+dt@xxxxxxxxxx> wrote:
> > >
> > > On Fri, Mar 26, 2021 at 9:20 AM Rob Clark <robdclark@xxxxxxxxx> wrote:
> > > >
> > > > On Fri, Mar 26, 2021 at 8:18 AM Rob Clark <robdclark@xxxxxxxxx> wrote:
> > > > >
> > > > > 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)
> > > >
> > > > what if we had a sort of multi-choice panel node:
> > > >
> > > >    panel: panel {
> > > >      compatible = "panel,one-of";
> > > >      compatible-one-of = "vendor1,panel-a", "vendor2,panel-b",
> > > > "vendor3,panel-c";
> > > >   };
> > > >
> > > > The kernel could construct power sequence timings that are the
> > > > superset of all the possible panels.  That seems about as explicit as
> > > > we could get in this sort of case.
> > >
> > > If we were to go this route, I'm inclined to say just shove all the
> > > possible panel compatibles into 'compatible'. That kind of breaks the
> > > notion of most specific to least specific. OTOH, it is saying the set
> > > of panels are somehow 'compatible' with each other.
> > >
> > > If there's not some level of compatibility between the panels, then
> > > it's still the bootloader's problem.
> > >
> >
> > I'm not sure about this.. since there could be slight differences in
> > various delay params between the possible panels.  I'd prefer that in
> > panel-simple.c, we listed exact delay params "vendorFoo,panelBar", but
> > it could mean that for a device that had three possible panels the
> > worst case (max of all possible delays) could be higher than any
> > individual choice.. and I don't think we should encourage the "white
> > lie" approach (which will be the obvious outcome of not handling this
> > directly in dt IME, based on prior art).  OTOH pushing it to the
> > bootloader, when the bootloader actually has to power up the panel
> > (and abide by the necessary delays) to figure out what choice we have
> > isn't a viable option either.
>
> I was only saying if the panels are different enough and there's not a
> worse case setting, then it's back to a bootloader problem. If we have
> multiple distinct compatibles, then it means the kernel should be able
> to figure out settings that work on all the possible panels listed.
>
> > It is better to be explicit about what we know and at the same time
> > about what we don't know.
>
> Can you be explicit about what we know and don't know here? With what
> you proposed and my alternative, at the end of the day we just have a
> list of compatibles. The only implicit part is the expectation that
> the set is somehow compatible with each other.
>

Ok, I think I was being incompatible with my definition of "compatible" ;-)

To make sure we are on the same page, this is what I have in mind:

1) the panels are compatible enough that if a user breaks their panel
and takes device in for repair, they might end up with a different
panel
2) the different possible panels may have different power-on delay,
etc, but max of all possible power on delays is fine and enough to get
the kernel to the point that it can probe EDID and figure out the rest

BR,
-R