linux-kernel - Re: [RFC] firmware loader: retry _nowait requests when userhelper is not yet available

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Message-ID: <CAAMvbhGBW3q8=b93kC2HZC_dCywznebycjYSe_zQtNNt49LbPA@mail.gmail.com>
Date:	Mon, 19 Mar 2012 12:41:32 +0000
From:	James Courtier-Dutton <james.dutton@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Christian Lamparter <chunkeey@...glemail.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
	alan@...rguk.ukuu.org.uk,
	Linux PM mailing list <linux-pm@...r.kernel.org>
Subject: Re: [RFC] firmware loader: retry _nowait requests when userhelper is
 not yet available

On 16 March 2012 23:37, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Fri, Mar 16, 2012 at 3:57 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
>> +       if (nowait) {
>> +               int limit = loading_timeout * MSEC_PER_SEC;
>> +               int timeout = 10;  /* in msec */
>> +
>> +               while (usermodehelper_is_disabled()) {
>> +                       read_unlock_usermodehelper();
>> +
>> +                       msleep(timeout);
>
> Ugh, this is disgusting.
>
> The whole point of nowait was that it's not synchronous - so it should
> just be driven by timers, not some kind of random "while sleep" loop.
>
> And that fw thing already does have a timeout associated with it, and
> quite frankly, the *sane* approach is to do all this not in
> _request_firmware(), but in request_firmware_work_func() - never even
> call _request_firmware() in the first place if the system isn't ready,
> just reset the timeout to retry it again later.
>
> Seriously. The rule should be really simple: nothing should *ever*
> call "request_firmware()" (or the _request_firmware() helper function)
> while the system is not up. That WARN_ON() should remain totally and
> utterly unconditional, and it should *not* be conditional on "nowait"
> or any idiotic crappy hack like that.
>
> If there is an asynchronous thread - and there is, for the _nowait()
> case - that asynchronous thread should set up the timer and retry in
> ten seconds or whatever. It should *not* call 'request_firmware()" and
> expect that to do something special.
>

Could an alternative be a new event/callback to the driver informing
it that the request_firmware() functionallity is present or not.
The driver would then get a callback on each transition from
"request_firmware() present" to/from "request_firmware() absent".
So, on boot up, the driver ".probe" gets called, and only when enough
infrastructure is up and running does the "request_firmware() present"
callback get called.
The question then becomes
Q1: "how do we determine when enough infrastructure is up for
request_firmware() to succeed?"
Now, if that question can actually be answered, I would prefer that
approach, than to each driver starting a timer based request, to keep
requesting every 2 seconds until it works.

Other similar themes:
could be the driver registers an interest in a particular firmware
file in a particular location, and then when the system determines
that that particular file is now readable, it makes a callback to the
driver. It is certainly an easier question to answer than Q1. No
timers needed.
This could be implemented with a "request_firmware(filename)" that
returns imeadiately but adds the filename to the list of firmware
files to look out for.
The driver then gets a callback "here_is_requested_firmware(filename)"
when the system determines that it can read it.
This list could be scanned immediately and after that; only when a
filesystem is mounted, or the user requests a rescan.

Kind Regards

James
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