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

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Message-ID: <557975C2.4090905@ahsoftware.de>
Date:	Thu, 11 Jun 2015 13:49:22 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	Linus Walleij <linus.walleij@...aro.org>
CC:	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	Grant Likely <grant.likely@...aro.org>,
	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>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	"linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
	"list@...dq.ahsoftware:DRM PANEL DRIVERS" 
	<dri-devel@...ts.freedesktop.org>, dmaengine@...r.kernel.org,
	Dan Williams <dan.j.williams@...el.com>,
	"linux-usb@...r.kernel.org" <linux-i2c@...r.kernel.org>
Subject: Re: [PATCH 00/21] On-demand device registration

Am 11.06.2015 um 13:24 schrieb Alexander Holler:
> Am 11.06.2015 um 12:17 schrieb Alexander Holler:
>> Am 11.06.2015 um 10:12 schrieb Linus Walleij:
>>> On Wed, Jun 10, 2015 at 10:28 AM, Alexander Holler
>>> <holler@...oftware.de> wrote:
>>>> Am 10.06.2015 um 09:30 schrieb Linus Walleij:
>>>
>>>>> 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.
>>>
>>> Sorry I don't get that. Care to elaborate on why?
>>
>> Because loading/initializing on demand doesn't give you any solved order
>> of drivers to initialize. And it can't because it has no idea about the
>> requirements of other drivers. The reason why it might work better in
>> the case of the tegra is that it might give you another initialization
>> order than the one which is currently choosen, which, by luck, might be
>> a better one.
>>
>> But maybe I missed something, I haven't looked at the patches at all.
>> But just loading on demand, can't magically give you a working order of
>> drivers to initialize. E.g. how do you choose the first driver to
>> initialize?
> 
> Other problems you will run into are time constraints and multithreaded
> drivers.
> 
> E.g. we all should know how tricky it sometimes is to avoid deadlocks.
> And with loading on demand, you are extending this problem over the
> initialization of maybe a whole bunch of other drivers which might be
> started by calling one function of another driver. And a function call
> might need a very long time to finish during which an unpredictable
> amount of things may happen.
> 
> It would make me wonder if that will end up with a good, usable and as
> simple as possible solution.

Besides that instrumenting every call to another driver in order to fix
a onetime operation (the initialization) sounds like an enormous
overhead. Initialization is done pnly once, regardless how long a system
runs, but the instrumentation to fix this onetime operation would slow
down the operation during the whole runtime of a system.

I don't think this is what should be done.

> 
> Regards,
> 
> Alexander Holler

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