breaking DT compatibility

[Andre Przywara]

Hi,

On 11/02/16 11:44, Mark Rutland wrote:
> On Thu, Feb 11, 2016 at 11:00:48AM +0100, Maxime Ripard wrote:
>> On Wed, Feb 10, 2016 at 04:30:01PM +0000, Mark Rutland wrote:
>>> On Wed, Feb 10, 2016 at 03:37:55PM +0100, Maxime Ripard wrote:
>>>> Hi Rob,
>>>>
>>>> On Wed, Feb 10, 2016 at 07:42:02AM -0600, Rob Herring wrote:
>>>>> On Wed, Feb 10, 2016 at 6:30 AM, Andre Przywara <andre.przywara at arm.com> wrote:
>>>>>> Hi,
>>>>>>
>>>>>> just a ping:
>>>>>>
>>>>>> Are we really OK with breaking existing DTs in 4.6? (per the code in
>>>>>> -next: f7d372ba54ea04d528a291b8dbe34716507bb60b, which is this patch).
>>>>>
>>>>> I only warn and make sure people are aware of the issue. I leave that
>>>>> up to platform maintainers to decide. It depends on the maturity of
>>>>> the platform and users.
>>>>
>>>> The impacted SoCs support is really partial. For the most supported
>>>> one, big things like display or sound are totally missing, and we
>>>> still update them on a regular basis to add support for new
>>>> devices. As such, users are very likely to upgrade the DT from one
>>>> version to another just because there's new devices available to
>>>> them. And the newest SoC impacted just got introduced in 4.5, and only
>>>> has the UART and MMC devices available.
>>>>
>>>>> If people complain about it then it's their mess. For platforms
>>>>> supported in distros such as debian or fedora, I would strongly
>>>>> recommend against breaking compatibility.
>>>>
>>>> None of them are officially supported:
>>>> https://www.debian.org/releases/stable/armhf/ch02s01.html.en
>>>> https://fedoraproject.org/wiki/Architectures/ARM#Fedora_23
>>>>
>>>> Only the older one are, and they are not affected by this patch.
>>>>
>>>>> They do ship dtbs, but it's a chicken and egg problem. If dtbs were
>>>>> stable and provided by firmware, then they wouldn't have to provide
>>>>> them. If dtbs are unstable, then they have no choice.
>>>>
>>>> And while it might work great on platforms that have all the needed
>>>> documentation, or a perfect one, which is our case. Almost each
>>>> release, we discover that something is not working as it was
>>>> documented, when it was documented in the first place.
>>>>
>>>> It also seems that even on well documented platforms, supported by the
>>>> vendors, the stable ABI dream is not going to happen:
>>>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/arm/Atmel/README#n105
>>>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/arm/marvell,berlin.txt#n4
>>>
>>> To be quite frank, I completely disagree with that stance.
>>>
>>> It seems like the only reason DT bindings aren't remaining stable is
>>> because people are deliberately ignoring the requirement and reasoning
>>> for doing so.
>>
>> And for DT maintainers saying on multiple occasions that it's bad but
>> ok to break it and / or that they never actually said that it was a
>> stable ABI...
> 
> Evidently there is a communication failure. Generally, the statement has
> been that old DTBs should continue to work. That's even documented, as
> Andre pointed out:
> 
> Documentation/devicetree/bindings/ABI.txt
> 
> There are obviously shades of grey, and _rarely_ it might be necessary
> to deliberately break a binding. However, that should be the rare
> last-resort case, rather than a crutch for development.
> 
> Saying "bad but ok" underplays the "bad" and overplays the "ok".
> 
>> I'm guessing it could be a stable ABI if there was bindings
>> reviews. Rob actually started to review a significant amount of
>> bindings lately, and that's really appreciated, but if you don't
>> review all the bindings, then we're going to make mistakes.
> 
> I agree that it is unfortunate that we cannot provide the level of
> review that we want to.
> 
> Mistakes will happen even with review; that doesn't necessitate removing
> support for a binding.
> 
>>> I agree that it can be painful, and that we cannot predict the future.
>>> There will always be bugs.
>>
>> In our case, we can't even predict the present.
>>
>>> Having code in mainline comes with responsibilities. One of those is to
>>> keep said code working for existing users. Otherwise, why bother having
>>> it in mainline at all?
>>
>> None of our existing users ever complained.
> 
> I believe that in this case, Andre was complaining about this particular
> breakage, unless I have misunderstood.
> 
> To be clear, I'm arguing for the strategy going forward. If no-one has
> complained about the stuff broken up to this point, let's not waste time
> restoring that.
> 
> Going forward we need to keep old DTBs supported.

FWIW: I had a chat with Maxime and we exchanged ideas on how to go on
from here, by reworking this commit to be both compatible and fixing the
issue we are having with the current PLL6 binding. I will look at
generating patches for this shortly.

So I guess we can move the discussion more to a code level from here on.

Cheers,
Andre.