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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Jan 19, 2016 at 05:04:33PM +0200, Marius Vlad wrote:
> 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?

I'd either keep it fairly generic, or document all of them. What you
had here seemed like something in between.

> 
> 
> > 
> > > + *
> > > + * |[!<-- 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



-- 
Ville Syrjälä
Intel OTC
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