Re: RFC: proposal to extend the open-pic interrupt specifier definition

[Grant Likely <grant.likely-s3s/WqlpOiPyB63q8FvJNQ@public.gmane.org>]

On Tue, Jan 12, 2010 at 11:36 AM, Yoder Stuart-B08248
<B08248-KZfg59tc24xl57MIdRCFDg@public.gmane.org> wrote:
>> > The advantage of the above approach is backwards compatibility.
>> > Existing interrupt specifiers (and device trees) are valid with
>> > this proposal.
>>
>> Actually they're not, like BenH pointed out.
>
> The proposal is backwards compatible.  An existing interrupt
> specifier  (e.g. interrupts = <24 2>;) retains its exact
> same meaning.  Old device trees do not need to change
> to comply with the proposal.

You also need to deal with the case of old drivers incorrectly binding
to and trying to understand the new data.

> I'm not directly familiar with the case Ben pointed out, but
> it sounded like Apple used the 1st cell in some non-standard
> way.
>
> It is true that openpic drivers would need to change to handle
> the new specifier-- minimally masking the level/sense field
> to 2 bits.

Which makes the new binding incompatible with old kernels/drivers
which just leads to confusion.  It's not worth toying with.  Just
create a new compatible value for this new binding and be done with
it.  When a driver gets modified to handle the new behaviour, then it
can be also changed to bind against the new compatible value too.

g.

-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.