[RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices to mach-omap2)

[Pantelis Antoniou]

Hi Grant,

On Nov 6, 2012, at 9:45 PM, Grant Likely wrote:

> On Tue, Nov 6, 2012 at 7:34 PM, Pantelis Antoniou
> <panto at antoniou-consulting.com> wrote:
>> On Nov 6, 2012, at 12:14 PM, Grant Likely wrote:
>>> On Tue, Nov 6, 2012 at 10:30 AM, Pantelis Antoniou
>>> <panto at antoniou-consulting.com> wrote:
>>>> For hot-plugging, you need it. Whether kernel code can deal with
>>>> large parts of the DT going away... How about we use the dead
>>>> properties method and move/tag the removed modes as such, and not
>>>> really remove them.
>>> 
>>> Nodes already use krefs, and I'm thinking about making them kobjects
>>> so that they appear in sysfs and we'll have some tools to figure out
>>> when reference counts don't get decremented properly.
>>> 
>> 
>> From the little I've looked in the of code, and the drivers, it's going
>> to be pretty bad. I don't think all users take references properly, and
>> we have a big global lock for accessing the DT.
> 
> I'm a lot more optimistic on this front... I wrote a patch today to
> make the change and took some measurements:
> 
> On the versatile express qemu model I measured the free memory with
> /proc/device-tree, with /sys/device-tree, and with both. Here's what I
> found:
> 
> /proc/device-tree only: 114776kB free
> /sys/device-tree only: 114792kB free
> both enabled: 114716kB free
> 
> The back of a napkin calculation indicates that on this platform
> /proc/devicetree costs 76kB and /sys/device-tree costs 60kb. I'm happy
> to see that using /sys instead of /proc appears to be slightly cheaper
> which makes it easier to justify the change. The diffstat makes me
> even happier:
> 
> arch/arm/plat-omap/Kconfig                |    1 -
> arch/powerpc/platforms/pseries/dlpar.c    |   23 -----------
> arch/powerpc/platforms/pseries/reconfig.c |   40 ------------------
> drivers/of/Kconfig                        |    8 ----
> drivers/of/base.c                         |  116
> ++++++++++++++++++++++++++++------------------------
> drivers/of/fdt.c                          |    5 ++-
> fs/proc/Makefile                          |    1 -
> fs/proc/proc_devtree.c                    |   13 +-----
> fs/proc/root.c                            |    4 +-
> include/linux/of.h                        |   35 ++++++++++++----
> include/linux/proc_fs.h                   |   16 --------
> include/linux/string.h                    |   11 +++++
> 12 files changed, 107 insertions(+), 166 deletions(-)
> 

Interesting. Not so bad then.

> There are still a few odds and ends that need to be tidied up, but
> I'll get it out for review shortly. I've not touched the sparc code
> yet, and I need to take another look over the existing OF_DYNAMIC
> code. I think that CONFIG_OF_DYNAMIC will probably go away and the add
> node/property functions will get used by fdt.c and pdt.c for initial
> construction of the device tree.

CONFIG_OF_DYNAMIC never made sense to me. Glad to see the config option
gone. I'm not totally up to date with the -next dt stuff, but if we're
there can we rename all the prom_ functions to something saner?

> 
>> Adding and removing nodes at runtime as part of the normal operation of
>> the system (and not as something that happens once in a blue moon under
>> controlled conditions) will uncover lots of bugs.
> 
> I'm hoping so! Its time to clean that mess up. :-) Fortunately adding
> nodes is not where we're going to have problems. The problems will be
> on node removal. Addition-only at least means we can have something
> useful before hunting down and squashing all the bugs.

I'll admit that removing nodes is going to be quite rare at least for
me use cases. I did come across a couple of people that do hot-plugging
(using something completely different) that could use it for sure.

> 
>> So let's think about locking too
> 
> Yes, the locking does need to be sorted out.
> 

Perhaps come up with a dt-stress test tool/boot time validator?

> g.

Regards

-- Pantelis

P.S. Lots of teeth grinding in the ELCE about the lack of a DT preprocessor.
The pinctrl arguments have been mentioned more than once.