[PATCH v4 1/4] dt-bindings: simplefb: Specify node location and handoff related properties

[Grant Likely]

On Thu, Nov 13, 2014 at 7:23 PM, Hans de Goede <hdegoede at redhat.com> wrote:
> Since simplefb nodes do not relate directly to hw typically they have been
> placed in the root of the devicetree. As the represent runtime information
> having them as sub-nodes of /chosen is more logical, specify this.
>
> Also specify when to set the chosen stdout-path property to a simplefb node.
>
> For reliable handover to a hardware specific driver, that driver needs to
> know which simplefb to unregister when taking over, specify how the hw driver
> can find the matching simplefb node.
>
> Last add some advice on how to fill and use simplefb nodes from a firmware
> pov.
>
> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> Acked-by: Geert Uytterhoeven <geert at linux-m68k.org>
> Acked-by: Maxime Ripard <maxime.ripard at free-electrons.com>
> --
> Changes in v2:
> -Add stdout-path to the example code
> Changes in v3:
> -Specify that the node name must be "framebuffer@<address>"
> -Specify that the link to link simplefb node and display hw nodes to one
>  another for handover goes into the simplefb node, and must be called "display"
> -Specify how aliases may be used to tell the OS how to number displays
> -Update the example to reflect these changes
> Changes in v4:
> -Keep the size of the reg property in the example as it was before
> ---
>  .../bindings/video/simple-framebuffer.txt          | 49 +++++++++++++++++++++-
>  1 file changed, 48 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/devicetree/bindings/video/simple-framebuffer.txt b/Documentation/devicetree/bindings/video/simple-framebuffer.txt
> index 8f35718..c548f33 100644
> --- a/Documentation/devicetree/bindings/video/simple-framebuffer.txt
> +++ b/Documentation/devicetree/bindings/video/simple-framebuffer.txt
> @@ -4,6 +4,31 @@ A simple frame-buffer describes a frame-buffer setup by firmware or
>  the bootloader, with the assumption that the display hardware has already
>  been set up to scan out from the memory pointed to by the reg property.
>
> +Since simplefb nodes represent runtime information they must be sub-nodes of
> +the chosen node (*). Simplefb nodes must be named "framebuffer@<address>".
> +
> +If a simplefb node represents the preferred console for user interaction,
> +then the chosen node's stdout-path property must point to it.

Since we've decided to point the display aliases at the display node
instead of the framebuffer, the stdout-path property should also point
to the display node. The simplefb driver can still work out which
framebuffer to use for stdout path in the same way that it figures out
how to resolve the display alias. This way the stdout details aren't
lost when the framebuffer is disabled and It also means that
stdout-path can be merely set to the alias ("display0") instead of the
full path ("/full/path/to/node").

> +
> +If the devicetree contains nodes for the display hardware used by a simplefb,
> +then the simplefb node must contain a property called "display", which
> +contains a phandle pointing to the primary display hw node, so that the OS
> +knows which simplefb to disable when handing over control to a driver for the
> +real hardware. The bindings for the hw nodes must specify which node is
> +considered the primary node.
> +
> +It is advised to add display# aliases to help the OS determine how to number
> +things. If display# aliases are used, then if the simplefb node contains a

s/then if/and if/

> +"display" property then the /aliases/display# path must point to the display
> +hw node the "display" property points to, otherwise it must point directly
> +to the simplefb node.
> +
> +It is advised that devicetree files contain pre-filled, disabled framebuffer
> +nodes, so that the firmware only needs to update the mode information and
> +enable them. This way if e.g. later on support for more display clocks get
> +added, the simplefb nodes will already contain this info and the firmware
> +does not need to be updated.

You should merge your guidance of naming pre-filled framebuffer nodes
into this patch. There is no need to have it separate.

> +
>  Required properties:
>  - compatible: "simple-framebuffer"
>  - reg: Should contain the location and size of the framebuffer memory.
> @@ -19,14 +44,36 @@ Optional properties:
>             are expected to already be configured correctly. The OS must
>             ensure these clocks are not modified or disabled while the
>             simple framebuffer remains active.
> +- display : phandle pointing to the primary display hardware node
>
>  Example:
>
> -       framebuffer {
> +aliases {
> +       display0 = &lcdc0;
> +}
> +
> +chosen {
> +       framebuffer0: framebuffer at 1d385000 {
>                 compatible = "simple-framebuffer";
>                 reg = <0x1d385000 (1600 * 1200 * 2)>;
>                 width = <1600>;
>                 height = <1200>;
>                 stride = <(1600 * 2)>;
>                 format = "r5g6b5";
> +               clocks = <&ahb_gates 36>, <&ahb_gates 43>, <&ahb_gates 44>;
> +               display = <&lcdc0>;
> +       };
> +       stdout-path = &framebuffer0;
> +};
> +
> +soc at 01c00000 {
> +       lcdc0: lcdc at 1c0c000 {
> +               compatible = "allwinner,sun4i-a10-lcdc";
> +               ...
>         };
> +};
> +
> +
> +*) Older devicetree files may have a compatible = "simple-framebuffer" node
> +in a different place, operating systems must first enumerate any compatible
> +nodes found under chosen and then check for other compatible nodes.
> --
> 2.1.0
>