[RFC 00/16] Armada DRM support for OLPC XO-1.75 laptop

[Lubomir Rintel]

On Wed, 2018-12-19 at 10:13 +0100, Daniel Vetter wrote:
> On Wed, Dec 19, 2018 at 9:40 AM Lubomir Rintel <lkundrak at v3.sk> wrote:
> > Hi,
> > 
> > here are patches that make the Armada DRM work on an OLPC XO-1.75.
> > They build on patches previously submitted by Russel King (included here for
> > completeness as well).
> > 
> > They certainly need some more love, but I'm not able to improve them until
> > I get to understand some things first. I'm posting a couple of questions
> > below in hope someone is kind enough to help me deal with my confusion.
> > 
> > The display pipeline on the laptop looks like this:
> > 
> >   Armada LCDC
> >   -----------
> >       |
> >       v
> > 
> >   Himax HX8837 "DCON"
> >   -------------------
> >   RGB input from LCDC
> >   Controlled via I2C
> >   Backlight control
> >   Can slow down the panel refresh rate to save power
> >   Optional dithering for color mode, "swizzling"
> >   Has DRAM attached, can freeze a single frame in it
> >   Doc: http://wiki.laptop.org/images/0/09/DCON_datasheet_HX8837-A.pdf
> >       |
> >       v
> > 
> >   Innolux LS075AT011 Panel
> >   ------------------------
> >   7.5", 1200x900
> >   No datasheet.
> >   http://wiki.laptop.org/go/Display
> >   Can opreate in two modes:
> > 
> >    R G B
> >    G B R ...   or   "e-book" daylight readable
> >    B R G            reflexive B&W
> >      .
> >      :
> > 
> > Here's what's not clear to me:
> > 
> > 1.) Is the Himax chip an encoder here?
> 
> Since it's an external IP probably better to model it as a bridge.
> 
> > 2.) What's the use of an encoder anyways? If a panel was connected directly
> >     do the RGB output from the LCDC, would we have to fake one? Is that the
> >     point of things like i.MX driver's drivers/gpu/drm/imx/parallel-display.c?
> 
> encoder is the thing between crtc and connector. It's exposed to
> userspace (which is a uapi design accident, but can't change that),
> hence why it's required and some drivers have dummy ones.
> 
> The real usage is purely internally for the atomic helpers.

I'm wondering how should the drm_encoder be instantiated? The imx uses
a dummy (compatible="fsl,imx-parallel-display") node. That almost
certainly counts as devicetree abuse and is a wrong thing to do.
Should the armada driver always create one? In that case the
transcoders that connect to it (just tda998x, I suppose) should cease
doing so and hook on a bridge instead.

Or should the driver for the Himax chip create an encoder and bridge
intance? Perhaps not, if my understanding is correct that drm_encoder
shall not be used for things outside the lcdc/crtc chip.

> Most
> callbacks in the encoder should be optional, so a dummy encoder should
> be a lot thinner than your imx example. The imx example should be
> using the panel-bridge I think, which would take care of most of the
> boilerplate (but panel-bridge didn't exist back then I guess).

> tda998x predates bridges, which is why there's still some
> encoder_helper usage in there. It's become a proper bridge driver now
> I think (but some drivers still use the old interface). All new
> transcoder chip drivers should be bridges.

Which drivers are good examples?

> Did you look at the
> kerneldoc for bridges? If those aren't clear we need to fix them.

Yes, if you mean this:
https://dri.freedesktop.org/docs/drm/gpu/drm-kms-helpers.html#bridges

If they weren't clear it might be entirely my fault. I'm totally new to
most of the stuff there and the sheer size of the structures and the
API is a tough challenge for my working memory.

The two important things that were not clear to me are essentially
what's discussed above. That is:

1.) Should I use this at all?
>From "These are handy when a regular drm_encoder entity isn’t enough to
represent the entire encoder chain." I wrongly concluded that I
probably don't heed to.

2.) Which driver creates the drm_encoder object and which one connects
it to the drm_bridge.

> > 4.) How shall I expose the fancy functionality of the Himax to the userspace?
> >     Notably, the freeze frame. The OLPC laptops with the stock firmware like
> >     to suspend the SoC very aggressively to save power (in 20 seconds of
> >     inactivity or so). If the display is open (it can also be turned around
> >     for a tablet or e-book mode), it makes sense to freeze the picture and
> >     keep the panel on, if the laptop is closed, we want to turn it off.
> >     Should the behavior be exposed via sysfs (as it is with the current OLPC
> >     kernels), or a DRM property? Would it require libdrm support?
> 
> I've accidentally seen how that's implemented for fbdev in the older
> olpc drivers, and that's definitely _not_ how we want to support this.
> Essentially what you have here is a fancy self-refresh panel. That's
> already fully supported by DRM uapi, so I'd say first implement that.
> Once we have that we can figure out a special property that tells the
> driver to not shut down the screen on suspend, but just go into
> self-refresh. Implementing the self-refresh stuff in DRM will be the
> big pile of work (since that's something which goes beyond the generic
> drm_bridge interface).

Thank you, this looks pretty helpful.

Currently the MMP2 platform barely boots with the mainline kernel, let
alone suspends and resumes. I guess it would be all right if the self-
refresh-on-suspend trick gets worked out later, when the OLPC patch
queue drains a bit.

How do the DRM properties get set? Do they need support in some
userspace library, perhaps libdrm?

> >   Himax HX8837 "DCON"
> >   -------------------
> >   RGB input from LCDC
> >   Controlled via I2C
> 
> These two shouldn't be a problem.
> 
> >   Backlight control
> 
> We already have backlight interfaces

Yes; this is already supported in the RFC version of the driver chained
to this message.

> >   Can slow down the panel refresh rate to save power
> 
> Just expose this as modes with different vrefresh (and do that
> modeswitch without doing a full modeset). Bonus if you do it
> automatically, which can be done with the self-refresh infrastructure.
> 
> >   Optional dithering for color mode, "swizzling"
> 
> Not exactly clear, but I'm assuming the bus_format stuff on bridges
> should help with this.
> 
> >   Has DRAM attached, can freeze a single frame in it
> 
> Classic self-refresh support. Noralf Tronnes has done great work here
> past year to improve the infrastructure and let drivers implement this
> with less typing. Also look at the recent work by vmwgfx folks.

Thanks for above the pointers.

> Cheers, Daniel

I appreciate your response very much.

I'll try to digest it over the holidays and hopefully figure out how to
follow up with an updated version sometime early next year.

If I'm unsuccessful perhaps I'll manage to take some poor graphics
hacker hostage at FOSDEM.

Take care,
Lubo