[yocto] question about variables/parameters

[Jeff Hagen]

I am completely confused about bitbake variables how and when to set 
them to get the desired result in the poky/yocto environment.

For context, I am just a user but I have been around awhile. I have been 
using yocto/poky for several years now. I have written a number of 
simple recipes and successfully deployed embedded linux builds on a 
number of custom projects and architectures (x86 x86_64 zync and 
alterasoc ). This stuff works. My hat is off to the architects.

The problem comes when I want to change something. Frankly, I dont 
understand the documentation. Its way too generic. I end up wandering 
around the recipes and web searching and trying everything until I find 
some seemingly random combination of bbappend or conf file or 
variable_name that works. Once found, it works no sweat, but there has 
to be a better way.

So here is an example of a problem I am trying to solve. I need to add a 
boot parameter to the kernel.
When I run bitbake -v -f core-image-minimal for an x86 system I notice 
that (at least in the old version I am using) uses syslinux as the boot 
agent.  So I look at syslinux it needs a file called syslinux.cfg. There 
is a parameter there called APPEND that I need to add the keyword to. 
Then I find the syslinux.cfg file in a yocto build and I see that its 
created by a python script inside of syslinux.bbclass. I look there and 
sure enough there is a big comment there telling me to set the APPEND 
variable for the class. I also notice that the python script that 
creates syslinux.cfg runs when I run bitbake -v -f core-image-minimal

Also in the documentation there is a class called syslinux and it lists 
the variables that I found in the comments bbclass file. This is no 
doubt some clever auto-doc feature.

But thats where it ends. How do I know how to set that APPEND variable 
for my custom build?
The answer is either a bbappend file in my layer, a conf file, or 
something I can put in local.conf.

Rather than just telling me the answer, can someone please describe the 
reasoning that would go into figuring it out so I can figure out other 
similar issues on my own later? Or perhaps this was already done and I 
am missing some documentation somewhere. Can you please direct me? 
Thanks for your patience.

Jeff Hagen

-- 
Jeffrey R Hagen
Lithe Technology LLC
jhagen at lithetechnology.com
(520) 488-1155 (mobile)
(520) 477-6066 (office)
http://www.lithetechnology.com