[PATCH 00/12] introduce support for early platform drivers

[Rob Herring]

On Mon, May 14, 2018 at 6:38 AM, Bartosz Golaszewski <brgl at bgdev.pl> wrote:
> 2018-05-11 22:13 GMT+02:00 Rob Herring <robh+dt at kernel.org>:
>> On Fri, May 11, 2018 at 11:20 AM, Bartosz Golaszewski <brgl at bgdev.pl> wrote:
>>> This series is a follow-up to the RFC[1] posted a couple days ago.
>>>
>>> NOTE: this series applies on top of my recent patches[2] that move the previous
>>> implementation of early platform devices to arch/sh.
>>>
>>> Problem:
>>>
>>> Certain class of devices, such as timers, certain clock drivers and irq chip
>>> drivers need to be probed early in the boot sequence. The currently preferred
>>> approach is using one of the OF_DECLARE() macros. This however does not create
>>> a platform device which has many drawbacks - such as not being able to use
>>> devres routines, dev_ log functions or no way of deferring the init OF function
>>> if some other resources are missing.
>>
>> I skimmed though this and it doesn't look horrible (how's that for
>> positive feedback? ;) ). But before going into the details, I think
>> first there needs to be agreement this is the right direction.
>>
>> The question does remain though as to whether this class of devices
>> should be platform drivers. They can't be modules. They can't be
>> hotplugged. Can they be runtime-pm enabled? So the advantage is ...
>>
>
> The main (but not the only) advantage for drivers that can both be
> platform drivers and OF_DECLARE drivers is that we get a single entry
> point and can reuse code without resorting to checking if (!dev). It
> results in more consistent code base. Another big advantage is
> consolidation of device tree and machine code for SoC drivers used in
> different boards of which some are still using board files and others
> are defined in DT (see: DaVinci).
>
>> I assume that the clock maintainers had some reason to move clocks to
>> be platform drivers. It's just not clear to me what that was.
>>
>>> For drivers that use both platform drivers and OF_DECLARE the situation is even
>>> more complicated as the code needs to take into account that there can possibly
>>> be no struct device present. For a specific use case that we're having problems
>>> with, please refer to the recent DaVinci common-clock conversion patches and
>>> the nasty workaround that this problem implies[3].
>>
>> So devm_kzalloc will work with this solution? Why did we need
>> devm_kzalloc in the first place? The clocks can never be removed and
>> cleaning up on error paths is kind of pointless. The system would be
>> hosed, right?
>>
>
> It depends - not all clocks are necessary for system to boot.

That doesn't matter. You have a single driver for all/most of the
clocks, so the driver can't be removed.

>>> We also used to have an early platform drivers implementation but they were not
>>> integrated with the linux device model at all - they merely used the same data
>>> structures. The users could not use devres, defer probe and the early devices
>>> never became actual platform devices later on.
>>>
>>> Proposed solution:
>>>
>>> This series aims at solving this problem by (re-)introducing the concept of
>>> early platform drivers and devices - this time however in a way that seamlessly
>>> integrates with the existing platform drivers and also offers device-tree
>>> support.
>>>
>>> The idea is to provide a way for users to probe devices early, while already
>>> being able to use devres, devices resources and properties and also deferred
>>> probing.
>>>
>>> New structures are introduced: the early platform driver contains the
>>> early_probe callback which has the same signature as regular platform_device
>>> probe. This callback is called early on. The user can have both the early and
>>> regular probe speficied or only one of them and they both receive the same
>>> platform device object as argument. Any device data allocated early will be
>>> carried over to the normal probe.
>>>
>>> The architecture code is responsible for calling early_platform_start() in
>>> which the early drivers will be registered and devices populated from DT.
>>
>> Can we really do this in one spot for different devices (clk, timers,
>> irq). The sequence is all very carefully crafted. Platform specific
>> hooks is another thing to consider.
>>
>
> This is why I added support for early probe deferral - so that we can
> stop caring for the order as long as the drivers are aware of other
> resources they need and we call early_platform_start() the moment the
> earliest of the early devices is needed.

Deferred probe helps for inter-device dependencies, but I am more
concerned about timing of trying to register clocksources, irqchips,
etc. What happens if we probe drivers before the infrastructure is
initialized?

Rob