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

From: Alexander Holler <holler@ahsoftware.de>
To: Linus Walleij <linus.walleij@linaro.org>,
	Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"linux-fbdev@vger.kernel.org" <linux-fbdev@vger.kernel.org>,
	linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
	open@wandq.ahsoftware, dmaengine@vger.kernel.org,
	"linux-gpio@vger.kernel.org" <linux-gpio@vger.kernel.org>,
	"linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
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	"linux-pwm@vger.kernel.org" <linux-pwm@vger.kernel.org>,
	linux-clk@vger.kernel.org,
	"linux-tegra@vger.kernel.org" <linux-tegra@vger.kernel.org>,
	list@wandq.ahsoftware, Rob Herring <robh+dt@kernel.org>,
	DRM PANEL DRIVERS <dri-devel@lists.freedesktop.org>,
	Grant Likely <grant.likely@linaro.org>,
	Dan Williams <dan.j.williams@intel.com>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	"linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>l
Subject: Re: [PATCH 00/21] On-demand device registration
Date: Wed, 10 Jun 2015 10:28:35 +0200	[thread overview]
Message-ID: <5577F533.1060007@ahsoftware.de> (raw)
In-Reply-To: <CACRpkdZuSj+S2x1yz6n5PgcRK=3ZCcq_To124b2Y+Oj54aydpg@mail.gmail.com>

Am 10.06.2015 um 09:30 schrieb Linus Walleij:
> 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.
>
> 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.

You would end up with the same problem of deadlocks as currently, and 
you would still need something ugly like the defered probe brutforce to 
avoid them. So what would you win with that instrumentation?

Alexander Holler
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