Re: [PATCH 00/21] On-demand device registration

[Alexander Holler <holler@ahsoftware.de>]

Am 10.06.2015 um 14:23 schrieb Andrzej Hajda:
> On 06/10/2015 12:19 PM, Tomeu Vizoso wrote:
>> On 10 June 2015 at 09:30, Linus Walleij <linus.walleij@linaro.org> wrote:
>>> On Tue, Jun 2, 2015 at 12:14 PM, Tomeu Vizoso
>>> <tomeu.vizoso@collabora.com> wrote:
>>>> On 2 June 2015 at 10:48, Linus Walleij <linus.walleij@linaro.org> wrote:
>>>
>>>>> This is what systemd is doing in userspace for starting services:
>>>>> ask for your dependencies and wait for them if they are not
>>>>> there. So drivers ask for resources and wait for them. It also
>>>>> needs to be abstract, so for example we need to be able to
>>>>> hang on regulator_get() until the driver is up and providing that
>>>>> regulator, and as long as everything is in slowpath it should
>>>>> be OK. (And vice versa mutatis mutandis for clk, gpio, pin
>>>>> control, interrupts (!) and DMA channels for example.)
>>>>
>>>> I understood above that you propose probing devices in order, but now
>>>> you mention that resource getters would block until the dependency is
>>>> fulfilled which confuses me because if we are probing in order then
>>>> all dependencies would be fulfilled before the device in question gets
>>>> probed.
>>>
>>> Sorry, the problem space is a bit convoluted so the answers
>>> get a bit convoluted. Maybe I'm thinking aloud and altering the course
>>> of my thoughts as I type...
>>>
>>> I guess there can be explicit dependencies for resources like this
>>> patch does, but another way would be for all resource fetch functions
>>> to be instrumented, so that you do not block until you try to take
>>> a resource that is not yet there, e.g.:
>>>
>>> regulator_get(...) -> not available, so:
>>> - identify target regulator provider - this will need instrumentation
>>> - probe it
>>>
>>> It then turns out the regulator driver is on the i2c bus, so we
>>> need to probe the i2c driver:
>>> - identify target i2c host for the regulator driver - this will need
>>>   instrumentation
>>> - probe the i2c host driver
>>>
>>> i2c host comes out, probes the regulator driver, regulator driver
>>> probes and then the regulator_get() call returns.
>>
>> Hmm, if I understand correctly what you say, this is exactly what this
>> particular series does:
>>
>> regulator_get -> of_platform_device_ensure -> probe() on the platform
>> device that encloses the requested device node (i2c host) -> i2c slave
>> gets probed and the regulator registered -> regulator_get returns the
>> requested resource
> 
> The downside of this solution is that it will not work without device
> tree or even without device dependencies not explicitly specified in
> device tree.

Solution for what? The goal isn't to search another fancy
registration/initialization algorithm.

So I wonder which problem that would solve at all. It doesn't give you
some deterministic initialization order nor does it (re)solve
dependencies (besides directly from one driver to another, but that
isn't enough), nor does it solve the problem of identifying drivers (the
other end of such an instrumented on-demand-initialization-call). So all
it would be is some fancy on-demand initialization without having solved
any problem.

Sorry if that sounds hard. Maybe I miss something. But I don't see  any
currently existing problem the above described solution would solve,
besides beeing something different (which shouldn't be the goal).

Alexander Holler
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