Jerry, thank you for your reply, please see below:
On Wed, Jul 23, 2008 at 2:46 PM, Jerry Van Baren gerald.vanbaren@ge.com wrote:
vb wrote:
One thing which (IMO anyway) slows down its acceptance is the way it handles relocating of itself into an arbitrary DRAM area. (Arbitrary meaning that it depends on many factors and the exact address can't be assigned ahead of time).
One thing (IMO anyway) that allows u-boot to Just Work[tm] is that it relocates itself to the best location available, automatically adjusting to the probed memory configuration.
There. Fixed that. ;-)
Sorry, I was not clear - it indeed is a great property of u-boot that it can run on any probed memory config. What I meant to say was that the way it relocates itself is not perfect, and this is why u-boot is not becoming more ubiquitous.
While this relocation is seamless for u-boot in its released form, it becomes a pain each time a module needs to be added (not necessarily for following release to the Open Source community, for instance used for inhouse installations which include tens of thousand units for some companies). If these added modules were not written in position independent manner (namely, using structures with multiple stage indirect pointers interleaved with data), the effort to make these modules work in u-boot is very exhausting.
You lost me here.
What is a module?
for the purposes of this discussion any .o file or any library of .o files.
How does a it relate to u-boot?
I need to extend u-boot with certain functionality available from those modules.
Why does it need to be relocated?
because the modules are now part of u-boot, they need to be relocated along with the rest of the image.
Why isn't it written in position independent manner?
because they come from different sources, not concerned with u-boot, and just work and need to be ported as is.
How is your relocation methodology going to fixup a module's data structures?
The relocation will adjust only pointers to absolute addresses stored in text/data segments.
What pointers are in your data structures? Why do they need to be relocated?
in some cases data structures include pointers to other data structures, etc. (This is called 'tree' btw, sorry couldn't resist :-)
[snip]
Gcc supports "proper" relocation, if only we knew how to make it work for all "reasonable" versions of gcc (and Grant's conclusion is that many versions of gcc in use today do *not* support the relocation). http://article.gmane.org/gmane.comp.boot-loaders.u-boot/36343/
well, if there is a pointer to a data element or to a function stored in a data structure, the .bin image is built and fixed using actual addresses set by the linker. How can gcc help here?
Grant Likely created a patch that did this in the 1.3.0 timeframe, but ended up reverting it due to toolset problems - it was a patch before its time.
To read up on the history, follow this search: http://search.gmane.org/?query=relocation&author=grant+likely&group=gmane.comp.boot-loaders.u-boot&sort=date&DEFAULTOP=and&xP=Zreloc%09Zchang%09Zgrant%09Zlike&xFILTERS=Gcomp.boot-loaders.u-boot---A
I will read up on this.
[snip]
I made some experiments, and this seems feasible, this could be done as a local customization, but I would much prefer to release it to the u-boot community and make it part of mainline - will you consider such a patch?
Thank you for reading this far, please let me know what you think,
On one hand, your proposal sounds fairly simple and independent of the gcc version. The primary cost would be extra build steps
- Link at TEXT_BASE
- Link at address 0
- Binary diff the two to find addresses that need relocation
- Rebuild with the diff table (compile the diff table and relink)
add here inserting code to process the table and modify pointers after relocation (which is already being done, but suing a different algorithm)
On the other hand, the skeptic in me says it ain't that easy. Go ahead, prove me wrong. ;-)
I am sure it is doable, I am just trying to understand if this would be accepted in general, because it can be done differently depending if one needs to push it upstream or not.
cheers, /vb
Vadim
Best regards, gvb