On Thu, Mar 01, 2007 at 09:01:24AM -0500, Jerry Van Baren wrote:
Hi all,
This is a Request for Advice.
First off, for those on both the u-boot and linuxppc-dev lists, sorry for cross posting. :-)
Git repo pointers... libfdt hacks: http://www.cideas.us/cgi-bin/gitweb.cgi?p=linux/libfdt.git;a=summary u-boot hacks: http://www.cideas.us/cgi-bin/gitweb.cgi?p=u-boot/u-boot-mpc83xx;a=summary
I have a proof-of-concept (actually, somewhat more than PoC) fdt command built using libfdt. Now I'm looking for advice on the best way to turn the PoC into Real Code[tm].
Current commands:
fdt address - set the base address of the blob fdt get <property> - get a property fdt print <node> - print the subtree starting at <node>[1]
Planned commands:
fdt set <property> <value> - set a property value (do we want an "=" as in <property> = <value>?) fdt ?synthesize? - create the "chosen" branch and optionally the u-boot variables branch. The OF code/calls currently in bootm would be moved to this command. I'm open to suggestions for a good subcommand name.
Other commands:
fdt node <name> - create a node that can then be populated with properties (have not thought about this in any detail yet).
[1] If <node> is actually a <property> (which is a usage error, but I expect will happen all the time), "fdt print" actually does a "fdt get <property>" This makes "fdt get" redundant and likely will make it go away.
Philosophy question primarily for David Gibson and Wolfgang Denk: What is the best way to integrate libfdt with u-boot? Currently it is in its own git repository. Options?
- Do we want to capture the source in the u-boot git repository? If
so, it becomes a snapshot and will require cross pushing/pulling between the libfdt repo and the u-boot repo or they will drift apart. However, it makes problems #1 and #2 (below) simpler to solve but causes drift.
- Not capturing libfdt in the u-boot repo makes it more difficult for
integrating it with and maintaining it in u-boot, I'm not sure how to actually do it in a useful/usable manner.
I think ultimately, you'll have to pull libfdt into the u-boot sources. libfdt is supposed to run in many possible environments, and there's no realistic way it can be compiled independently of thost environments.
That said, any changes to the guts of libfdt that you need should go upstream (in the end; right now, there are bureaucratic problems with that, more below). That should reduce things to a one way pull, with some trivial tweaks to integrate with the u-boot environment.
There are three problem areas with libfdt:
- The official Makefile is stand-alone which doesn't work well with
u-boot. I took the expedient route for the PoC of simply replacing it with a u-boot style Makefile from a different lib* subdirectory. There should be a better way.
I think it might well be necessary to replace the Makefile for building libfdt into other packages. It would be nice to avoid that if possible, but a sensible method is not obvious to me.
- The official libfdt uses two header files that are not in u-boot. I
"fixed" this by substituting u-boot headers with equivalent functionality.
- I need to address this and see what the best compromise for header
files is... a) If the u-boot headers are acceptable for the stand-alone version of libfdt, that would be the simplest. b) It may be more effective to add the necessary linux headers to u-boot. c) We could use #ifdefs to conditionally include the right files. (but does u-boot have a distinctive configuration def? Probably...)
Ok, the way this is supposed to work is that the environment into which you're building should provide a replacement version of libfdt_env.h. The supplied version of libfdt_env.h is just for userland builds. u-boot's version will obviously use u-boot headers instead of standard library headers.
I should provide a comprehensive list of what libfdt_env.h needs to provide, but I haven't gotten around to it. From memory it's basically just the fixed-with integer types, a smallish subset of the str*() and mem*() functions, and the endian conversion functions.
I've really tried to keep libfdt's environment dependencies down, so I suggest you just start with an empty libfdt_env.h and add stuff based on the error messages until the compiler stops whinging about undefined things. It shouldn't take long.
- I added a "fdt_next_tag()" function to fdt_ro.c to allow me to step
through the blob's tags: uint32_t fdt_next_tag(const void *fdt, int offset, int *nextoffset, char **namep);
This is similar to "_fdt_next_tag()" (a "private" function, note the leading underscore) in fdt.c, but with a related but different purpose - the "_fdt_next_tag()" steps through _node_ tags (skipping property tags) where I need to step through all tags sequentially.
Um... no. _fdt_next_tag() steps through all tags (how else could it be used internally to find properties...?). If you really need this, we can change the function to be exported, which I've considered before. However, what are you using this function for? I had some node and property traversal functions on the drawing board.
Usability trivia for David: libfdt distinguishes between nodes and properties, which makes sense since that is how the fdt is structured. From a usability standpoint, however, it is annoying to have to separate the property name from the node, find the node, then find the property. I will probably create Yet Another Function: int fdt_split(char *path, char **property); Call it with a path string and the function will separate it into the node portion and the property name. If the path is invalid, it will return an error. If the path is a node, it will set **property to NULL and return the node's offset. If the path is a property, it will return the owning node's offset and set the **property pointer to point to the start of the property portion of the path (i.e. the next character after the last '/').
I don't like combining property and node name into a single path, because technically the property names occupy a different namespace from subnode names. Insane though it is, there exist some Apple device trees where a node has both a property and a subnode of the same name.