Hi Rob,

Am Freitag, 31. August 2018, 14:18:36 CEST schrieb Rob Herring:

On Wed, Aug 22, 2018 at 02:02:13PM +0200, Heiko Stuebner wrote:

quoted

Since at least 2014 coreboot exports board specific variant ids for
board-revision, used ram-modules and component variants on the same board
into the loaded devicetree.

These are set on all devicetree-based Chromebooks since then, so at
least we can make the effort to document these long-used properties.

Long used, but never reviewed, so that doesn't really matter.

quoted

A case where these are used is for example to determine the touchscreen
type that is only identifyable via the sku-id when updating its firmware
on the Scarlet tablet from the Gru ChromeOS family.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
---
 Documentation/devicetree/bindings/firmware/coreboot.txt | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Documentation/devicetree/bindings/firmware/coreboot.txt b/Documentation/devicetree/bindings/firmware/coreboot.txt
index 4c955703cea8..cfc7623e2577 100644
--- a/Documentation/devicetree/bindings/firmware/coreboot.txt
+++ b/Documentation/devicetree/bindings/firmware/coreboot.txt

@@ -21,6 +21,12 @@ Required properties:
 	0xc0389481 that resides in the topmost 8 bytes of the area.
 	See coreboot's src/include/imd.h for details.
 
+Board variant properties determined via strapping measures (like gpios):
+ - board-id: board-specific id indicating the board-revision
+ - ram-code: board-specific id identifying the used ram-module
+ - sku-id: board-specific id indicating a variant (using different
+           display panels for example)

The appear to be consumed by coreboot, but the purpose of the /firmware 
nodes has describing firmware interfaces provided by the platform. 

Not saying we can't put things to configure the firmware there, but it 
would be a departure and something we should consider. These properties 
aren't really coreboot specific and probably belong at the root node. 
Though I think we already discussed a 'board-id' property for QCom (and 
ended up with a compatible string approach instead.

These are not for configuring the firmware. Coreboot is reading the values
from hardware-strappings, like special gpios and inserts the properties into
the devicetree for the kernel or userspace to read back if needed.

So coreboot loads a devicetree without them from the boot-partition and
amends that devicetree we these properties.

As indicated above, devices since 2014 do that, so I thought it might make
sense to document that behaviour.


Heiko