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

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Message-ID: <557AC040.40803@ahsoftware.de>
Date:	Fri, 12 Jun 2015 13:19:28 +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>,
	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 12.06.2015 um 09:25 schrieb Linus Walleij:
> On Thu, Jun 11, 2015 at 6:40 PM, Alexander Holler <holler@...oftware.de> wrote:
>> Am 11.06.2015 um 14:30 schrieb Linus Walleij:
>
>>> Certainly it is possible to create deadlocks in this scenario, but the
>>> scope is not to create an ubreakable system.
>>
>> IAnd what happens if you run into a deadlock? Do you print "you've lost, try
>> changing your kernel config" in some output hidden by a splash-screen? ;)
>
> Sorry it sounds like a blanket argument, the fact that there are
> mutexes in the kernel makes it possible to deadlock, it doesn't
> mean we don't use mutexes. Some programming problems are
> just like such.

I'm not talking about specific deadlocks through mutexes. I'm talking 
about what happens when driver A needs driver B which needs driver A. 
How do you recognise and handle that with your instrumented on-demand 
device initialization? Such a circular dependency might happen by just 
adding a new fucntion call or by changing the kernel configuration. And 
with the on-demand stuff, the possibility that the developer introducing 
this new (maybe optional) call will never hit such a circular dependency 
is high. So you will end up with a never ending stream of problem 
reports whenever someone introduced such a circular dependecy without 
having noticed it.

And to come back to specific deadlocks, if you are extending function 
calls from something former simple to something which might initialize a 
whole bunch of drivers, needing maybe seconds, I wouldn't say this is a 
blanket argument, but a real thread.

Alexander Holler
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