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

[Tomeu Vizoso <tomeu.vizoso@collabora.com>]

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 I'm currently looking at is that an explicit dependency
graph would be useful to have for other purposes. For example to print
a neat warning when a dependency cannot be fulfilled. Or to refuse to
unbind a device which other devices depend on, or to automatically
unbind the devices that depend on it, or to print a warning if a
device is hotplugged off and other devices depend on it.

> This requires instrumentation on anything providing a resource
> to another driver like those I mentioned and a lot of overhead
> infrastructure, but I think it's the right approach. However I don't
> know if I would ever be able to pull that off myself, I know talk
> is cheap and I should show the code instead.

Yeah, if you can give it a second look and say if it matches what you
wrote above, it would be very much appreciated.

> Deepest respect for your efforts!

Thanks!

Tomeu

> Yours,
> Linus Walleij
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