Re: tile property contents

[Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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On Tue, Oct 14, 2014 at 01:23:22PM +1000, Dave Airlie wrote:
> Hi,
> 
> So I've been hacking on mutter and the gnome pieces for tiling, and
> I've at least fixed mutter locally so maximise windows works and the
> heads are in the right order.
> 
> Now I've strung all the pieces together using a single KMS property
> that X.org propogates, and mutter picks up and propagates over dbus as
> well,
> 
> Currently I've ascii encoded the property into a blob,
> 
> <ver>:<tileid>:<flags>:<maxhtiles>:<maxvtiles>:<h_tile_loc>:<v_tile_loc>:<tile_w>:<tile_h>
> 
> I'm thinking of dropping the version field and just exposing TILE2
> property if we need it later to add more values,
> 
> The other fields:
> tileid: a group id assigned by the kernel to all tiles in the same
> group - unique per group
> flags: bit 0 : single monitor enclosure
> maxhtiles: total number of horiz tiles
> maxvtiles: total number of vert tiles
> h_tile_loc: horiz location of this output in tile group
> v_tile_loc: vert location of this output in tile group
> tile_w: width of this tile
> tile_h: height of this tile.
> 
> Now we extract all of these from the DisplayID v1.3 block, and I'm
> wondering if maybe I shouldn't just export the whole DisplayID tiling
> info block instead, it however encodes a few other pieces of
> information, including bezel info, and some flags specifying behaviour
> in some cases.
> 
> The former could be more suitable for cases where DisplayID isn't
> available (Dual DSI panels?) but I'm worried abuot exposing too little
> at this point making TILE useless when the next monitor comes out.

I don't think this is a good fit to describe dual DSI panels in the
first place. While one of the modes (left-right split) could probably be
described using the above, the other mode (odd-even split) is more
difficult. In the latter mode, one controller will provide the odd lines
and the other controller will provide the even lines.

Also exporting the details about tiling presumes that each of the tiles
can work pretty much independently, too. That's not necessarily the case
for dual DSI. For a symmetric left-right split configuration this may be
somewhat true, at least for one of the halves. The second half can't
operate standalone. For an odd-even split I don't think either half can
be made to work standalone. I'm also not sure how left-right split
configurations work in video mode. I can imagine that both are really
needed for the panel to properly sync, since only the right half gets
the HBLANK and VBLANK signals.

One other thing that worries me about this is that we defer handling of
these complex configurations to userspace. I suppose this is fine, and
in fact the only way, if there is no knowledge about the tile layout in
kernel space. But if we know precisely how these various tiles are
connected, wouldn't we be better off abstracting this away within the
kernel and expose a single connector that is the union of all the tiles?
After all that's what the kernel is, an abstraction between hardware and
userspace.

Thierry

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