linux-kernel - Re: Regulator probe on demand (or circular dependencies)

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Date:   Mon, 9 Dec 2019 17:03:38 +0000
From:   Kieran Bingham <kieran.bingham@...asonboard.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Jacopo Mondi <jacopo@...ndi.org>,
        Niklas Söderlund <niklas.soderlund@...natech.se>
Subject: Re: Regulator probe on demand (or circular dependencies)

Hi Mark,

Thanks for getting back to me,

On 09/12/2019 16:37, Mark Brown wrote:
> On Fri, Dec 06, 2019 at 04:38:04PM +0000, Kieran Bingham wrote:
> 
>> The MAX9286 also exposes 2 GPIO pins, as such I have configured the
>> MAX9286 driver [1] to expose a gpio-chip [2].
> 
> So this seems like a MFD then?  The nice thing about using the MFD
> subsystem is that it means that the drivers for the various subsystems
> on the device can instantiate in any order and defer separately without
> interfering with each other which seems like it's the issue here.

Well that's part of the problem... the V4L2 async framework can not
currently support the device performing a probe-defer at all, so it
*will* fail later (and crash currently).

I hope we can fix this sometime - but it's a recurring pain point it
seems. Unless it's just our video-capture driver, I'll have to dig
deeper here, and check with Niklas.


>>  - is there anything I can do here within regulator_dev_lookup() to
>>    attempt creating the regulator_dev 'on-demand' when
>>    of_find_regulator_by_node(node) returns empty? (or is that crazy, and
>>    just a rabbit-hole?)
> 
> This seems like a terrible idea, you'll have a half baked regulator in
> the system which will need special casing all over the place and
> doubtless be an ongoing source of bugs.

Thanks - that's essentially what I'm glad to hear /before/ going down
some rabbit hole. I'll re-evaluate with the team, and see what the next
best steps are.

-- 
Regards
--
Kieran