Hi, On Fri, Mar 16, 2018 at 06:36:01PM +0530, Srivatsa Vaddagiri wrote: > Hello, > I'd like comments on some changes to fdt_overlay_apply() that I have > prototyped to allow merging of two or more overlay blobs (overlay blob is > considered as a DT blob having unresolved external references listed in > __fixups__ section). > > fdt_overlay_apply() API currently allows for an overlay DT blob to be merged > with a base blob. It assumes that all external symbols specified in overlay > blob's __fixups__ section are found in base blob's __symbols__ section and > aborts on the first instance where a symbol could not be found in base blob. > > This is mostly fine as the primary use of overlay is on a target for its > bootloader to merge various overlay blobs based on h/w configuration detected? > > We are exploring an extended use of fdt_overlay_apply() for offline use, i.e on > the host (build machine) side, which requires merging two overlay DT blobs i.e > the base (overlay) blob will not resolve all the external node references found > in overlay blob. > > Currently all the device-tree (DT) code for a given soc is maintained in a > common kernel repository. For example, this common DT code will have code for > audio, video, fingerprint, bluetooth etc. Further this DT code is typically > split into a base (soc-common) code and board specific code, with the soc code > being compiled as soc.dtb and board specific code being compiled as respective > overlay blobs (board1.dtbo, board2.dtbo etc). soc.dtb and boardX.dtbo files are > flashed separately on target (besides improving the overall size of DT blobs > flashed on target, Android Treble also requires separation of soc and board DT > bits). Bootloader will pick one of the board overlay blobs and merge it with > soc.dtb, before booting kernel which is presented a unified DT blob (soc + board > overlay). > > For ease of code maintenance and better control over release management, we are > exploring allowing some of the tech teams (audio/fingerprint sensor etc) to > maintain their kernel code (including their DT code) outside a common kernel > repository. In our experience, this simplifies number of branches maintained in > core kernel repo. New/experimental features in fingerprint sensor driver for > example that needs to be on a separate branch will not result in unnecessary > branching in core kenrel repo, affecting all other drivers. > > In addition to compiling DT code outside core kernel tree, we also want to merge > the blobs back to respective blobs found in kernel build tree (soc.dtb or > boardX.dtbo), as otherwise relying on bootloader to do all the overlay impacts > boot-time. > > This brings up the need to merge two overlay blobs (fingerprint-overlay.dtbo + > boardX.dtbo), which currently doesn't seem to be supported and which this patch > series aims to support. If I understood you right, you want in your "feature" overlay to depend on nodes that are only defined in your board overlay? If so, why not simply merge the __symbols__ node when we apply an overlay with the base tree, and just call fdt_apply_overlay as many times as needed? Maxime -- Maxime Ripard, Bootlin (formerly Free Electrons) Embedded Linux and Kernel engineering https://bootlin.com
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