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

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Date:	Thu, 11 Jun 2015 10:15:04 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
Cc:	Mark Rutland <mark.rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	dmaengine@...r.kernel.org,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
	linux-clk@...r.kernel.org,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	"open list:DRM PANEL DRIVERS" <dri-devel@...ts.freedesktop.org>,
	Grant Likely <grant.likely@...aro.org>,
	Alexander Holler <holler@...oftware.de>,
	Dan Williams <dan.j.williams@...el.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>
Subject: Re: [PATCH 00/21] On-demand device registration

On Wed, Jun 10, 2015 at 12:19 PM, Tomeu Vizoso
<tomeu.vizoso@...labora.com> wrote:
> On 10 June 2015 at 09:30, Linus Walleij <linus.walleij@...aro.org> wrote:

>> 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

Yes. But only for device tree.

> 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.

Unbind/remove() calls are the inverse usually yes.

But also the [runtime] power up/down sequences for the
devices tend to depend on a similar ordering or mostly
the same. (Mentioned this before I think.)

>> 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.

Yes you are right. But what about ACPI, board files,
Simple Firmware and future hardware description languages...

Yours,
Linus Walleij
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