[RFC PATCH 0/4] drivers/base: Generic framework for tracking internal interfaces

[Andrzej Hajda]

Hi Greg,

Thanks for comments. I CC Laurent, I hope it could be interesting for 
him also.

Greg Kroah-Hartman wrote, On 30.04.2014 17:49:
> On Wed, Apr 30, 2014 at 04:02:50PM +0200, Andrzej Hajda wrote:
>> Generic framework for tracking internal interfaces
>> ==================================================
>>
>> Summary
>> -------
>>
>> interface_tracker is a generic framework which allows to track appearance
>> and disappearance of different interfaces provided by kernel/driver code inside
>> the kernel. Examples of such interfaces: clocks, phys, regulators, drm_panel...
>> Interface is specified by a pair of object pointer and interface type. Object
>> type depends on interface type, for example interface type drm_panel determines
>> that object is a device_node. Object pointer is used to distinguish different
>> interfaces of the same type and should identify object the interface is bound to.
>> For example it could be DT node, struct device,...
>>
>> The framework provides two functions for interface providers which should be
>> called just after interface becomes available or just before interface
>> removal. Interface consumers can register callback which will be called
>> when requested interface changes its state, if during callback registration
>> the interface is already up, notification will be sent also.
>>
>> The framework allows nesting calls, for example it is possible to signal one
>> interface in tracker callback of another interface. It is done by putting
>> every request into the queue. If the queue is empty before adding
>> the request, the queue will be processed after, if there is already another
>> request in the queue it means the queue is already processed by different
>> thread so no additional action is required. With this pattern only spinlock
>> is necessary to protect the queue. However in case of removal of either
>> callback or interface caller should be sure that his request has been
>> processed so framework waits until the queue becomes empty, it is done using
>> completion mechanism.
>>
>> The framework functions should not be called in atomic context. Callbacks
>> should be aware that they can be called in any time between registration and
>> de-registration, so they should use synchronization mechanisms carefully.
>> Callbacks should also avoid to perform time consuming tasks, the framework
>> serializes them, so it could stall other callbacks.
>>
>> Use cases
>> ---------
>>
>> The interface is very generic, there are many situations it can be useful:
>>
>> 1. Replacement for subsystem specific methods of publishing/unpublishing
>> interfaces. Currently many frameworks allows only querying for presence of given
>> interface. In such cases client can defer probing or implement interface
>> polling, which is usually subobtimal. Additionally subsystems often do not
>> provide methods to signal to the consumers that they are about to be destroyed.
>>
>> 2. Monitoring for different interfaces provided by the same object. An example
>> should explain it better. Lets assume in device tree drm crtc device node have
>> video link to another node, so it knows only that there is something connected
>> to its RGB output. It can be a RGB panel (drm_panel framework), it can be an
>> image enhancer (SoC specific framework) or it can be some signal converter
>> (drm_encoder, drm_bridge, drm_encoder_slave...). Driver have only phandle to
>> another node. Currently it is difficult to handle such situations in a generic
>> way. interface_tracker should make it simple: crtc should monitor all supported
>> interface types that appears at the device_node pointed by the phandle.
>>
>> Potential use cases
>> -------------------
>>
>> Points mentioned above were the reasons for writing this framework. During
>> development I have realized that this framework can be also useful for other
>> tasks.
>>
>> 3. Replacement for deferred probing - if for some reason driver do not wants to
>> defer but it requires given resources it can use interface_tracker. It should be
>> possible to create an helper which will wait for appearance of all interfaces
>> from a given list, and 'continue' probe only when all resources becomes
>> available.
>>
>> 4. Replacement or helper for subsystem specific solutions:
>> - V4L2 subdev async registration,
>> - component framework.
>> Both frameworks solves a problem of tracking sub-components (un-)registration
>> by master device, it should be possible to do the same with interface_tracker
>> framework. Some additional helpers can be convienent to make the implementation
>> easier.
>>
>> 5. Cure the situation with sysfs 'unbind' property. Currently many drivers are
>> providers of different resources/interfaces: regulators, clocks, phys,
>> V4L2 subdevs, ... They are usually protected from module unloading by getting
>> module reference, but there is no protection from driver unbinding using sysfs
>> method: echo 'device' >/sys/bus/.../drivers/.../unbind. After unbind consumer
>> stays with the pointer to non-existing object, next time it tries to use it
>> system usually crashes. interface_tracker do not add any protection, but it adds
>> a way to signal to the consumer that given resource will disappear. It allows
>> to handle such situations more gently.
>>
>> Potential issues/extensions
>> ---------------------------
>>
>> 1. Scalability - the framework serializes all tasks and callbacks. In case there
>> are no many users it should not be a problem. If the number of users grows there
>> are different options to consider:
>> - run callbacks in parallel, I guess async_schedule_domain can be helpfull,
>> - partition trackers, for example per interface types - different interface
>>    types will use different internal queues/lists.
>>
>> 2. Complication of code - asynchronous programming usually seems to be more
>> complicated. Adding some helpers could make it less painfull.
>>
>> 3. Object comparison - currently object pointers are compared by value, it could
>> be desirable to allow also other ways of comparison, for example string
>> comparison. It is not a problem to extend the framework.
>>
>> TODO
>> ----
>>
>> 1. Testing - the patchset have not been tested yet with multiple users.
>
> That's not good :)

I would treat this series as a real RFC, with code which can be easily 
tested by volunteers but as for RFC I am asking for comments if the idea 
is OK and the patchset have chances to be accepted.
Anyway, I have performed some basic tests and it works :)

>
> What's wrong with the existing container code in the driver core, or the
> component code?  Shouldn't either of those work properly for you?  Or
> the managed token interface being proposed, how does that tie in here?

Container code seems to solve very specific problem, not really relevant 
to this patchset. The component code would solve a very specific part of 
problems this framework solves as described in use cases above. On the 
other side problems which components solves are
solvable with this framework.

Managed token interface is about sharing 'named' resources it is not 
connected with interface tracking.

>
>> 2. Add tracker support in other frameworks - currently there is only drm_panel.
>>     I plan also to add something more complicated, for example use it in
>>     exynos_drm to track components. If there is positive feedback I can try
>>     to add also other frameworks.
>> 3. devm_* registration.
>> 4. Helpers - as the situation 'wait for number interfaces before continue'
>>     seems to be quite common, some helper to easy handling it could be useful.
>>
>> Final remarks
>> -------------
>>
>> Primarily I have planned notifications for DRM panels. Next I have realized
>> something similar would be necessary for drm_bridge. Discussions with other
>> developers showed to me that it could be useful in many other areas. I am not
>> sure if other developers agree with adding it to things like regulators, clocks,
>> phys, but I will be glad if it can be used at least with drm_panel.
>
> Shouldn't the component code handle your DRM panels today?

I should correct my last sentence to this:
I will be glad if it can be used at least with video interfaces, ie. 
drm_panel, video encoders/bridges, image enhancers.

The main problem with component framework is that componentization 
significantly changes every driver and changes it in a way which is not 
compatible with traditional drivers, so devices which are intended to 
work with different DRM masters are hard to componentize if some of DRMs 
are componentized and some not.

If you look at interface_tracker framework it achieves the same goal
as componentization without even touching panel drivers, look at 2nd 
patch of the patchset - it adds interface_tracker support for all panels 
with 7 lines of code in drm_panel framework and of course the
change is backward compatible.
Adding interface_tracker support to the 'master' requires more work:
1. registration of the callback.
2. unregistration of the callback.
3. the callback code.
But it is still much less than in case of components.

And interface_tracker framework allows much more, as I described in "Use 
cases" and "Potential use cases" sections.

Regards
Andrzej

>
> thanks,
>
> greg k-h
>