Re: [PATCH i-g-t 2/2] lib/igt_kms: Short description between Intel/DRM terminology.

[Marius Vlad <marius.c.vlad@intel.com>]

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On Fri, Jan 15, 2016 at 02:46:45PM +0200, Ville Syrjälä wrote:
> On Fri, Jan 15, 2016 at 11:06:42AM +0200, Marius Vlad wrote:
> > lib/igt_kms: Briefly describe Intel-to-DRM mapping between pipes, encoders and
> > connectors.
> > 
> > Signed-off-by: Marius Vlad <marius.c.vlad@intel.com>
> > ---
> >  lib/igt_kms.c | 82 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 82 insertions(+)
> > 
> > diff --git a/lib/igt_kms.c b/lib/igt_kms.c
> > index c7a0b77..caa8837 100644
> > --- a/lib/igt_kms.c
> > +++ b/lib/igt_kms.c
> > @@ -68,6 +68,88 @@
> >   * functions have all igt_ prefixes. This part is still very much work in
> >   * progress and so also lacks a bit documentation for the individual functions.
> >   *
> > + * Intel/DRM terminology and display connections:
> > + *
> > + * Intel documentation describes the road from memory to an output connector as
> > + * follows:
> > + *
> > + * |[!<-- language="C" -->
> > + * .--------.     .-------.     .-------------.     .-----.
> > + * | Memory |---->| Pipes |---->| Transcoders |---->| DDI |
> > + * '--------'     '-------'     '-------------'     '-----'
> > + * ]|
> > + *
> > + * Pipes represent the front-end of the display contain the planes, blending,
> > + * and color correction, while the transcoders contain timing generators,
> > + * encoders, A/V mixers and PSR (Panel-Self Refresh) controllers. Finally the
> > + * DDI represent the connectors attached to the display.
> > + *
> > + *
> > + *  In DRM we have the following:
> > + *
> > + * |[!<-- language="C" -->
> > + * .-----------.     .-------.     .-----------.     .------------.
> > + * | Framebuff |---->| Pipes |---->| Encoders  |---->| Connectors |
> > + * '-----------'     '-------'     '-----------'     '------------'
> 
> "Framebuff"?
> 
> Also should this say crtc instead of pipe?
Right, pipe indeed.
> 
> > + * ]|
> > + *
> > + *
> > + * The frame buffer ties a reference to a memory object and provides a pointer
> > + * to the actual data.
> > + *
> > + * The pipe is used to set the display mode, timings and gamma tables. On some
> > + * hardware models this is tied with the transcoder.  In DRM-parlance this is
> > + * referred as a CRTC.
> > + *
> > + * Each pipe has multiple planes. On older hardware these planes where known as
> > + * primary plane, overlay/sprite plane, and cursor plane. From GEN9 (SKL/BXT)
> > + * each pipe has three planes and a cursor plane.
> 
> Not quite true.
> 
> > Each plane can be used as a
> > + * primary, as a sprite or as an overlay plane. The planes are the ones that
> > + * retrieve the pixels from memory and pushes them to the encoder.
> > + *
> > + * A pipe prior to GEN9:
> 
> Really more like g4x-bdw. Before g4x it was totally different, and gen4 was
> sort of mix of both the old and the new. And vlv/chv have two sprites on
> each pipe.
> 
> So given all the variations in the hardware, and the fact that it keeps
> changing all the time, I'm not convinced there's any point in
> documenting this in igt. It'll get stale real quick, and there are
> efforts to decouple igt from i915 as much as possible, so next thing you
> know someone else will want to docuemnt their favorite hardware here as
> well.
> 
> So if we do want to document this stuff, then I think it should be
> somewhere in the kernel modeset code (around crtc/plane init I suppose).

Yes, it might make more sense to be in the kernel. What I wanted to achieve
with this small intro was to bridge the gap between DRM terminology and
the one in Intel 01.org PRM for igt. For a first timer is rather tedious to
identify software/hadware abstraction when looking at the PRMs and when
looking in igt -- I know for a fact that I'm still having trouble
identifying those parts.

Daniel has already suggested to add some documention between
DRM-to-Intel Mappings in the kernel.

Do you suggest that we should document all platforms?


> 
> > + *
> > + * |[!<-- language="C" -->
> > + * .--------.
> > + * | Memory |             .--------.
> > + * |        |------------>| Cursor |------>...
> > + * |        |             '--------'
> > + * |        |
> > + * |        |         .--------.
> > + * |        |-------->| Sprite |---------->...
> > + * |        |         '--------'
> > + * |        |
> > + * |        |      .---------.
> > + * |        |----->| Primary |------------>...
> > + * |        |      '---------'
> > + * '--------'
> > + * ]|
> > + *
> > + * A pipe with universal planes:
> > + *
> > + * |[!<-- language="C" -->
> > + * .--------.
> > + * | Memory |             .--------.
> > + * |        |------------>| Cursor |------>...
> > + * |        |             '--------'
> > + * |        |
> > + * |        |         .-------.
> > + * |        |-------->| Plane |---------->...
> > + * |        |         '-------'
> > + * |        |
> > + * |        |      .-------.
> > + * |        |----->| Plane |------------>...
> > + * |        |      '-------'
> > + * '--------'
> > + * ]|
> > + *
> > + * The encoder is used to convert, depending on the output, pixels from pipes
> > + * to signals understood by the connector.
> > + *
> > + * The connector will connect to the output display. This contains information
> > + * about the attached display such as EDID, DPMS and information about modes
> > + * supported by the display.
> > + *
> >   * Note that this library's header pulls in the [i-g-t
> >   * framebuffer](intel-gpu-tools-Framebuffer.html) library as a
> >   * dependency.
> > -- 
> > 2.6.2
> > 
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx@lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 
> -- 
> Ville Syrjälä
> Intel OTC

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