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?
How do you specify optional and required but otherwise unrelated
extensions for a configuration?
};
};
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?
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?
Why can't we simply write the compatible constraint into the overlay
definition (fdto-#) and enumerate the overlays in the configuration?
On some busses detection is problematic. Two alternative add-ons may use
the same SPI address but need different FDT overlays.
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
