Re: [RFC] MPIC Bindings and Bindings for AMP Systems

On 12/23/2010 04:33 PM, Grant Likely wrote:

This argument has been rehashed many times, but it basically comes
down to compatible values should ideally be anchored to a real
implemented device, not to a family of devices, or to an unversioned
specification.

In practise, the implementation doesn't actually look any different
except that the 'reference' version specifies a specific
implementation instead of a generic name.  To use a concrete example,
if there are two parts using this MPIC, like the freescale p2040 and
p4080, and say for argument that the p2040 was implemented first, then
the compatible values would look like:

for the p2040:   compatible = "fsl,p2040-msgr";
for the p4080:   compatible = "fsl,p4080-msgr", "fsl,p2040-msgr";

and the driver could simply bind on "fsl,p2040-msgr" to work with both
chips.  So, instead of an arbitrary "fsl,mpic-msgr" string,
"fsl,p2040-msgr" is used as the baseline value and there is no
ambiguity about which part it describes.

If the p4080 is actually subtly different from the p2040, then
it would not claim compatibility with the former and the driver can
match against either string; adapting its behaviour as necessary.

Thanks for the explanation.  I see now that there is a warning note in 
'http://www.devicetree.org/Device_Tree_Usage' about this case.  Perhaps 
a "best practices" guide for writing bindings might be useful as well. 
I would be happy to contribute to that, but I am still learning the best 
practices :)

-- 
Meador Inge     | meador_inge AT mentor.com
Mentor Embedded | http://www.mentor.com/embedded-software