[yocto] How Does Yocto Fill In UBOOT_MACHINE?

[Saul Wold]

On 08/15/2013 11:15 AM, Leo Schwab wrote:
> Two months ago I never heard of Yocto -- now I are one.  I'm
> attempting to create a layer and pile of recipes that will allow us to
> adapt Freescale's i.MX support to our custom board.  Right now we're
> starting out with one of their evaluation boards, but will eventually
> transition to our own.  In anticipation of arriving hardware, I
> created a new config for u-boot by copying an existing config and
> renaming it.  The new config builds properly, but I don't know why.
> Grepping through the Yocto sources hasn't been very enlightening, and
> I was hoping I could get some insights into what's going on.
>
> I will try to be as succinct as I can (some of the names have been
> changed to protect the innocent).  We initially started by using the
> Freescale-furnished build targets directly.  I created a new layer
> (we'll call it meta-belis3), and in it created this
> conf/machine/ourboard.conf (apologies in advance for inappropriate
> line wrapping; GMail offers no control over this):
>
> ----
> #@TYPE: Machine
> #@NAME: ourboard
> #@DESCRIPTION: Based on machine configuration for Freescale i.MX6DL SABRE SD
>
> include conf/machine/include/imx-base.inc
> include conf/machine/include/tune-cortexa9.inc
>
> SOC_FAMILY = "mx6:mx6dl"
>
> PREFERRED_PROVIDER_u-boot = "u-boot-imx"
>
> UBOOT_MACHINE = "mx6dl_sabresd_config"
> UBOOT_SUFFIX = "bin"
> UBOOT_PADDING = "2"
>
> SERIAL_CONSOLE = "115200 ttymxc0"
>
> MACHINE_FEATURES += " pci wifi"
> ----
>
> This is basically a copy of Freescale's
> meta-fsl-arm/conf/machine/imx6dlsabresd.conf file, with trivial
> changes.  Inside the build directory's conf/local.conf, I set MACHINE
> to 'ourboard'.  This has worked for several weeks now.
>
> Yesterday, following Freescale's docs, I started making mods to u-boot
> to create a separate build target for our board.  The result is that I
> now have a working u-boot build target named 'mx6_ourboard_config".
> With that done, I created a .bbappend file to make the target
> available (in a file named
> meta-belis3/recipes-bsp/u-boot/u-boot-imx_2009.08.bbappend):
>
> ----
> FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
>
> SRC_URI_append_ourboard = "file://u-boot-ourboard.patch"
>
> UBOOT_MACHINE_ourboard = "mx6_ourboard_config"
> ----
>
> I then ran 'bitbake u-boot', and was rewarded with a u-boot image
> built for 'ourboard' -- the logs confirm it was built using the target
> mx6_ourboard_config.
>
> ...Except I never told it to build 'ourboard'; I haven't modified
> conf/machine/ourboard.conf yet.  It should still be building
> mx6dl_sabresd_config.  So UBOOT_MACHINE is magically picking up a
> different value.  If I remove my .bbappend file, then it resumes using
> the target mx6dl_sabresd_config.
>
But you did indirectly, I have to assume that your default MACHINE is 
set for "ourboard" and what's happened here is when the bbappend was 
added, your setting of UBOOT_MACHINE_ourboard is an override so it built 
using that setting.

> So:  What are the rules governing how UBOOT_MACHINE acquires its final
> value?  What did I neglect to consider?  Is my .bbappend bogus?  Is
> there some magic setting buried somewhere else I need to be aware of?
> Thank you in advance for all suggestions.
>
The magic is the OVERRIDES mechanism in base.bbclass, so the _ourboard 
is a machine override and takes presence over the default setting, in 
this case of the UBOOT_MACHINE.

You can see this in action by running a bitbake -e (prints the variable 
contents and how they were determined).

Sau!

> Schwab
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