linux-kernel - Re: [RFC 00/12] On-demand device registration

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Date:	Wed, 29 Apr 2015 11:46:43 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Stéphane Marchesin <marcheu@...omium.org>,
	Olof Johansson <olof@...om.net>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Mark Rutland <mark.rutland@....com>
Subject: Re: [RFC 00/12] On-demand device registration

Am 29.04.2015 um 08:58 schrieb Tomeu Vizoso:
> On 28 April 2015 at 20:17, Alexander Holler <holler@...oftware.de> wrote:
>> Am 28.04.2015 um 14:49 schrieb Tomeu Vizoso:
>>>
>>> On 25 April 2015 at 01:15, Alexander Holler <holler@...oftware.de> wrote:
>>>>
>>>> Am 24.04.2015 um 16:47 schrieb Tomeu Vizoso:
>>>>>
>>>>> Hi,
>>>>>
>>>>> while reading the thread [0] that Alexander Holler started with his
>>>>> series to make probing order deterministic, it occurred to me that it should
>>>>> be possible to achieve the same by probing devices as they are referenced by
>>>>> other devices.
>>>>>
>>>>> This basically reuses the information that is already embedded in the
>>>>> probe() implementations, saving us from refactoring existing drivers or
>>>>> adding information to DTBs.
>>>>>
>>>>> The main issue I see is that the registration code path in some
>>>>> subsystems may not be reentrant, so some refactoring of the locking will be
>>>>> needed. In my testing I have found this problem with regulators, as the
>>>>> supply of a regulator might end up being registered during the registration
>>>>> of the first one.
>>>>>
>>>>> Something I'm not completely happy with is that I have had to move the
>>>>> population of the device tree after all platform drivers have been
>>>>> registered. Otherwise I don't see how I could register drivers on demand as
>>>>> we don't have yet each driver's compatible strings.
>>>>>
>>>>> I have done my testing on a Tegra124-based Chromebook, and these patches
>>>>> were enough to eliminate all the deferred probes.
>>>>
>>>>
>>>> First you have to solve a problem which is totally unrelated to DT or
>>>> ACPI or x86 or ARM:
>>>>
>>>> I think as long as drivers don't register themself whithout any side
>>>> effect, this problem isn't solvable. In order to make an ordered list of
>>>> drivers to start, you need to know which drivers are actually available.
>>>
>>>
>>> Yeah, I kind of side-stepped that issue by waiting until all drivers
>>> have been registered before registering devices. I think someone
>>> suggested doing so in your thread (maybe Grant?).
>>
>>
>> That doesn't work. As said above, several drivers doing a lot more than just
>> registering in their initcall. They might even crash if some prerequisits
>> aren't given. And several of these prerequisits (init orders) are hardcoded
>> by various means.
>
> But aren't those dependencies being taken care currently by the
> initcall level the driver is placed in? That remains the same in this
> approach.

In short, no. There are various very ugly things done in several drivers 
to enforce some order.

Regards,

Alexander Holler
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