need some clarification on how environments work

[Sascha Hauer]

On Wed, Feb 08, 2012 at 08:19:18AM +0000, Premi, Sanjeev wrote:
> ----Original Message-----
> > From: barebox-bounces at lists.infradead.org 
> > [mailto:barebox-bounces at lists.infradead.org] On Behalf Of 
> > Robert P. J. Day
> > Sent: Wednesday, February 08, 2012 5:42 AM
> > To: U-Boot Version 2 (barebox)
> > Subject: need some clarification on how environments work
> > 
> > 
> >   ok, i'm trying to figure out how environments work so bear with me
> > as i'm going to ask a number of questions.  i'm reading this page:
> > 
> > http://wiki.barebox.org/doku.php?id=user:environment
> > 
> > and based on that, i took a look at what i have running on my beagle
> > xM having configured and built *entirely* using defaults.
> > confusingly, i have no /dev/env0 as that web page seems to suggest.
> > here's what's in my dev directory:
> > 
> > barebox at Texas Instrument's Beagle:/ ls -l dev
> > cr-------- 4294967295 zero
> > crw-------  134217728 ram0
> > crw-------       7496 defaultenv
> > crw------- 4294967295 mem
> > crw-------       1024 twl4030
> > crw------- 3963617280 disk0
> > crw-------  123346944 disk0.0
> > 
> >   so i have a "defaultenv" file there.  i puzzled over this for a
> > while until i noticed that there is no arch/arm/boards/beagle/env/
> > directory, as there is for most of the other boards.
> > 
> >   so let's step back and look at the relevant source in
> > common/Makefile:
> > 
> > =====
> > 
> > ifdef CONFIG_DEFAULT_ENVIRONMENT
> > $(obj)/startup.o: include/generated/barebox_default_env.h
> > $(obj)/env.o: include/generated/barebox_default_env.h
> > 
> > ifeq ($(CONFIG_DEFAULT_ENVIRONMENT_GENERIC),y)
> > DEFAULT_ENVIRONMENT_PATH = "defaultenv"
> > endif
> > 
> > ifneq ($(CONFIG_DEFAULT_ENVIRONMENT_PATH),"")
> > DEFAULT_ENVIRONMENT_PATH += $(CONFIG_DEFAULT_ENVIRONMENT_PATH)
> > endif
> > 
> > ENV_FILES := $(shell cd $(srctree); for i in
> > $(DEFAULT_ENVIRONMENT_PATH); do find $${i} -type f -exec readlink -f
> > '{}' \;; done)
> > 
> > endif # ifdef CONFIG_DEFAULT_ENVIRONMENT
> > 
> > barebox_default_env: $(ENV_FILES)
> >         $(Q)$(srctree)/scripts/genenv $(srctree) $(objtree)
> > $(DEFAULT_ENVIRONMENT_PATH)
> > 
> > =====
> > 
> >    as well as the menu selections from "make menuconfig":
> > 
> >       [*] Compile in default environment
> >       [*]   Default environment generic
> >       ()    Default environment path
> > 
> > so now, the questions.
> > 
> >   first, what is the "default" environment?  is that the one found
> > for the board in arch/arm/boards/<board>/env/config?  so that there is
> > *no* current default environment for the beagleboard right now?
> > 
> >   also, from the Kconfig help text:
> > 
> > CONFIG_DEFAULT_ENVIRONMENT:
> > 
> > Enabling this option will give you a default environment when
> > the environment found in the environment sector is invalid
> > 
> >   is that true?  as in, the "default" environment will only be used if
> > what's in the environment sector is found to be invalid?
> > 
> >   next, what is meant by the "generic" default environment?  from the
> > Kconfig file,
> > 
> > "With this option barebox will use the generic default
> > environment found under defaultenv/ in the src tree.
> > The Directory given with DEFAULT_ENVIRONMENT_PATH
> > will be added to the default environment. This should
> > at least contain a /env/config file.
> > This will be able to overwrite the files from defaultenv."
> > 
> >   ok, i can appreciate that -- it's a "generic" environment, defined
> > by the file defaultenv/config, and it's apparently what i got, is that
> > it?  but why the reference to DEFAULT_ENVIRONMENT_PATH, which is an
> > entirely separate Kconfig option?
> > 
> >   and regarding the final setting, CONFIG_DEFAULT_ENVIRONMENT_PATH,
> > how does that affect the first two settings?  i'm just trying to
> > puzzle all this out.
> 
> I have recently finished creating the default env for omap3evm; so
> will be able to share my uderstanding. Hope to send the patches by
> end of the day (atleast an RFC, if I am unable to boot linux) OR
> may be earlier.
> 
> CONFIG_DEFAULT_ENVIRONMENT appears to control whether "any" default
> env should be included in the barebox. Haven't tried env from NAND
> device (hopefully mounting as /dev/env0) so far, but I assume from
> boot sequence that I observe that env from NAND will be used and
> there is no fallback.
> 
> CONFIG_DEFAULT_ENVIRONMENT_GENERIC leads to inclusion of a generic
> "template" env into barebox - not really useful, but atleast you
> have a template to work with and al the FIXMEs included there will
> get you started.
> 
> If CONFIG_DEFAULT_ENVIRONMENT_PATH is used the "specific" path
> you have specified, will now be used for the default env. And it
> overrides the "generic" env path. So now you have a board specific
> env.

This is not the whole truth. CONFIG_DEFAULT_ENVIRONMENT_PATH can
be a list of directories. All pathes in this list are copied into
a temporary directory. This means that you can compose your environment
out of different directories. This also means that you can overwrite
files with more specific versions. If CONFIG_DEFAULT_ENVIRONMENT is enabled
'defaultenv' will be added to the start of the list.
So defaultenv/ contains a 'config' files with FIXMEs. This is normally
overwritten with the contents of your board specific config file
in arch/<arch>/<board>/env/config.

Sascha

-- 
Pengutronix e.K.                           |                             |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |