Re[2]: Where did find_bus() go in 2.6.18?

[Paul Sokolovsky <pmiscml@gmail.com>]

Hello Greg,

Monday, November 20, 2006, 2:12:12 AM, you wrote:

> On Mon, Nov 20, 2006 at 12:45:51AM +0100, Jiri Slaby wrote:
>> Paul Sokolovsky wrote:
>> >   But alas, the commit message is not as good as some others are, and
>> > doesn't mention what should be used instead. So, if find_bus() is
>> > "unused", what should be used instead?
>> 
>> You should probably mention what for?

> Exactly.  It was removed because no one was using it, and I couldn't
> think of a reason why it would be needed.

  So, I assume the answer to my question is "No replacement. Ability
to query bus set in the kernel was just removed, period." That's to
which conclusion I came myself after studying 2.6.18 source and patch
from 2.6.17.

[Uninteresting specific case]

  Ok, so the situation is following: we have a kind of multi-layered
driver here. Lowest level is a w1_slave bus driver, talking to a
specific chip and providing low-level API for accessing data in terms
of this chip (or chip class) notions. Above it, we have higher-level
driver which interprets data from the low-level one, converting it to
a standard device-independent form, plus possibly does some other
minor things, like providing feedback indication on these data.
(Forgot to say that this is battery driver.)

  So, just in case if some reader of this has quick suggestion of
merging these drivers into one, thanks, but they do different things,
and we want to keep them nicely decoupled. But now issue of how these
drivers talk between themselves raises, and that's exactly the grief
point.

  High-level driver used to find w1 bus by name, then enumerate
devices on the bus, to find compatible device on it, then hooks into
that device and its driver.

  So, you see the issue: we cannot enumerate devices on w1 bus. (And
yes, w1_bus_type is not exported).

  Sure, the source is up:
http://handhelds.org/cgi-bin/cvsweb.cgi/linux/kernel26/drivers/misc/h2200_battery.c?rev=1.20&content-type=text/x-cvsweb-markup


[Trends thoughts]

  But note that I don't really ask mainline kernel developers how to
fix this driver - I would actually be ashamed to do so, as I myself a
(newbie) kernel hacker. So, the question stays the same, though I
probably reformulate it a bit stronger now: how it came that ability
to query buses (at all) was removed from 2.6.18?

  As it was before, it was clear that LDM consists of multiple layers,
and each layer offers consistent and complete set of operations on it,
like adding new object on this layer, removing, adding child, removing
child, *and* query objects on this level or among childs. I may miss
some accidental gaps in that picture of course, but it still was an
integral, complete design paradigm offering full dynamicity and
introspection.

  And suddenly - oops, in 2.6.18 we lose ability to query the highest
level of hierarchy, namely bus set. And on what criterion? "unused". I
would really dream that such core, the most basic APIs are not being
defined in terms of "someone does use it right now". A method to query
objects of core kernel data sets is just integral part of interface to
these datasets, you cannot remove it and not cripple such interface.
Again, it's loss of introspection, and that's not just "cleanup", it's
a paradigm shift.

  Suddenly, concrete building of LDM appears to be shaking. And reasonable
question here is: is this a trend? What we will lose next - ability to
query/enumerate devices on bus? How much time it will take until
someone submit patch like "You know, I had a look at this
device vs drivers matching in LDM. You know, I found that in 90% it's
one-to-one mapping. You know, I think that rest 10% are just doing
something wrong. You know, let's don't care about them. Here's a
patch to match compile-time by C symbols. Oh no, wait, I have a patch
in preparation which kills device vs drivers distinction at all,
leading us to ol' good dirty monolithic 'drivers'." And it may be the
case that even designers of LDM will look at the code after all
those previous "cleanups" applied, and would agree that such patch
comes as pretty natural succession.


  Thanks for following my mental movie ;-). It's not a pure rhetoric
though, I indeed would like to know the answer - are all those
structures in LDM are big guys' games only? And would it better for small
guys to be as KISS as stupid, err..., possible? ;-)

> Also, any reason why your drivers aren't in the mainline kernel yet?  It
> would have kept something like this from happening a while ago.  And, it
> will also help out with the recent driver core changes that are being
> planned and are starting to show up in the -mm tree.  If your stuff is
> in the kernel, then I'll do the work for you, otherwise, you all are on
> your own :(

  One reason is of course because it's not that easy to get something
into mainline. ;-) Another one is that we have great deal of code made by
different people over different periods of time, and we need to do
internal cleanup first. And for example, as new people do it, and
learn the entire codebase, they find that some things are done,
well, so to say, above the ground level. And they start to worry if
that was right in the first place, and would it be now possible to get
this somebody's code into mainline, or its better to keep doing
homework and reapply the KISS paradigm. It is also a bit of ethical
problem, as gentlemen who made such implementations, were apparently
experienced developers to come up with such solutions, but no longer
around/have resources to lobby for such code themselves, and newcomers
would likely get only frustration from trying to do that, especially
if they see there *is* at least an abstract possibility to do it in
some other, more mundane way. And yet they can't readily do that local
paradigm shift, as well, they don't have enough experience to undo
what gurus did previously, plus the whole Linux-on-PDA community is
too short on people to spend time flip-flopping each other's work.

  All above has little to do with mainline kernel, of course, and not
its problem in any way. Just another entry into your sociological
survey "Why people don't submit code upstream." ;-)

> thanks,

> greg k-h


Thanks for your following this case,

-- 
 Paul                            mailto:pmiscml@gmail.com

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