Re: connector - encoder relationship

[Sascha Hauer <s.hauer@pengutronix.de>]

On Wed, May 25, 2011 at 12:07:33PM -0400, Alex Deucher wrote:
> On Wed, May 25, 2011 at 4:36 AM, Sascha Hauer <s.hauer@pengutronix.de> wrote:
> > Hi All,
> >
> > I'm currently looking into implementing a SoC graphics core with KMS.
> >
> > What I wonder about is the relationship between connectors and encoders.
> > On my board I have a sii9022 HDMI encoder connected via i2c. This chip
> > supports reading edid data, tracks the connection status of the display
> > and needs to be configured when the resolution changes. So my first
> > guess is that I have to implement an encoder. looking at struct
> > drm_encoder_funcs there are no callbacks suitable for this. struct
> > drm_connector_funcs on the other hand has all the callbacks I need.
> > Now I look at drivers/gpu/drm/i2c. The drivers there implement a
> > drm_encoder_slave which also has all the callbacks I need. So from
> > one point of view the functions should be implemented in the connector,
> > while in the i2c case it's implemented in the (slave-) encoder.
> > Generally I have the feeling that my sii9022 should be both an encoder
> > and a connector, but this doesn't fit into the current scheme.
> >
> > I'm confused. Could someone clarify this a bit?
> 
> The general idea is this:
> framebuffer -> crtc -> encoder -> connector -> monitor
> 
> - The framebuffer is just a buffer vram that has an image encoded in
> it as an array of pixels.
> - The crtc reads the data out of the framebuffer and generates the
> video mode timing in conjunction with a PLL.  The crtc also determines
> what part of the framebuffer is read; e.g., when multi-head is
> enabled, each crtc scans out of a different part of vram; in clone
> mode, each crtc scans out of the same part of vram.
> - The encoder takes the digital bitstream from the crtc and converts
> it to the appropriate analog levels for transmission across the
> connector to the monitor.
> - The connector provides the appropriate physical plug (HDMI, DVI-D,
> VGA, S-video, etc.) for the monitor to connect to.
> 
> crtcs are usually routeable and can be connected to one or many
> encoders simultaneously.  The encoders can also be connected to one or
> more connectors (e.g., you might have a single TMDS encoder that is
> connected to the digital portion of a DVI-I port and also to a HDMI
> port, or you might have a DAC that is connected to a VGA port and an
> S-video port).  Finally i2c lines are associated with connectors,
> since generally the i2c pins are wired to the connector and in many
> cases, each connectors has it's own i2c line.  For example, you might
> have a single TMDS encoder that is wired to a DVI-D port with its own
> i2c line and an HDMI port with its own i2c line.  Each connector may
> have it's own hotplug (HPD) pin as well.  That way you could query the
> EDID from each connector separately for example.
> 
> Is the sii9022 being driven by the output of a DVO encoder from your
> display hw?  If so your display path would look like:
> 
> crtc -> DVO (master encoder) -> sii9022 (slave encoder) -> HDMI plug -> monitor
> 
> The general GPU driver might register the i2c buses that are used by
> the various parts (encoder, connector), then it's just a matter of
> plugging in the right parts for each component (e.g. you'd hook up the
> sii9022 i2c control bus to the DVO encoder, and the ddc i2c bus to the
> hdmi connector object, etc.).
> 
> I hope this helps.

Thanks, that helped a lot. There are most probably more questions but
for the moment I'll continue digging myself through the maze ;)

Thanks
 Sascha

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