Hi Heinrich,
On Tue, Dec 12, 2023 at 3:43 PM Heinrich Schuchardt xypron.glpk@gmx.de wrote:
On 12.12.23 15:05, Simon Glass wrote:
Hi,
The devicetree files for a board can be quite large, perhaps around 60KB. To boot on any supported board, many of them need to be provided, typically hundreds.
All boards for a particular SoC share common parts. It would be helpful to use overlays for common pieces, to reduce the overall size.
Some boards have extension add-ons which have their own devicetree overlays. It would be helpful to know which ones should be applied for a particular extension.
I propose implementing extensions in FIT. This has a new '/extensions' node, so you can specify what extensions are available for each FIT configuration.
For example:
/ { images { kernel { // common kernel };
fdt-1 { // FDT for board1 }; fdto-1 { // FDT overlay }; fdto-2 { // FDT overlay }; fdto-3 { // FDT overlay };};
configurations { conf-1 { compatible = ... fdt = "fdt-1"; extensions = "ext1", "ext-2";
Shouldn't there be braces around the item list?
I don't think so.
How do you specify optional and required but otherwise unrelated extensions for a configuration?
Do we actually need to know which extensions are required or optional? Do you have an example?
};};
extensions { ext-1 { fdto = "fdto-1"; // FDT overlay for this 'cape' or 'hat' kernel = "kernel-1"; compatible = "vendor,combined-device1"; extensions = "ext-3"; };
ext-2 { fdto = "fdto-2"; // fdt overlay for this 'cape' compatible = "vendor,device2"; }; ext-3 { fdto = "fdto-3"; compatible = "vendor,device3"; };}; };
So FIT configurations have a list of supported extensions. The extensions are hierarchical so that you can have ext-1 which can optionally have ext-2 as well. This allows boards to share a common
ext2 seems not to be related to ext-3. Do you mean ext-3 optionally extending ext-1? How would you specify that ext-n is required for ext-m to work?
Yes, I mean "optionally extending ext2". Again, I don't consider required extensions here.
The sequence of applying overlays may make a difference. I cannot see that your current suggestion defines a sequence in which the overlays are applied.
SoC to add in overlays as needed by their board. It also allows common 'capes' or 'hats' to be specified only once and used by a group of boards which share the same interface.
Within U-Boot, extensions actually present are declared by a sysinfo driver for the board, with new methods:
get_compat() - determine the compatible strings for the current platform get_ext() - get a list of compatible strings for extensions which are actually present
Do you expect U-Boot to have code that injects device-tree fragments with a compatible string into the control FDT after discovering add-ons?
Yes, that's right, by applying overlays.
Why can't we simply write the compatible constraint into the overlay definition (fdto-#) and enumerate the overlays in the configuration?
Yes, that is the example quoted below.
On some busses detection is problematic. Two alternative add-ons may use the same SPI address but need different FDT overlays.
If there is no way to detect the extension, then it cannot work anyway, right?
BTW I am not suggesting that the bus is used for detection, although I suppose this is possible. More likely there are GPIOs which can be decoded to indicate the extension, or perhaps an I2C EEPROM.
Best regards
Heinrich
The extension compatible-strings are used to select the correct things from the FIT, apply the overlays and produce the final DT.
To make this simpler for the common case (without extensions), we can allow multiple FDT images for a configuration, with the first one being the base SoC .dtb and the others being the .dtbo overlay(s) for the board:
confi-1 { compatible = ... fdt = "fdt-1", "fdto-1"; };
Comments welcome.
Regards, Simon