linux-kernel - Re: A workaround for request_firmware() stuck in module

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Date:	Thu, 6 Sep 2012 13:59:57 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Takashi Iwai <tiwai@...e.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Kay Sievers <kay@...y.org>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: A workaround for request_firmware() stuck in module_init

> Sorry, I don't see anyone explained clearly why request_firmware()
> can't be called inside module_init() in module case, so maybe it is
> a bit early to say it is a fix on 'bug', :-)

Because the firmware load may trigger a need to load a driver to load the
firmware.

> > dev_discard_firmware()
> >
> > so you an instance can drop its firmware reference if it doesn't need it
> > post probe.
> 
> This kind of mechanism has been implemented already: request_firmware()
> and release_firmware() will get and put a refcount.  And, the reference
> count is associated with firmware name, and it should be so, IMO.

Yes - so a dev_ firmware interface is very thin.

> > You broke suspend/resume for lots of devices.
> 
> The firmware cache mechanism will keep the firmware during suspend/resume
> cycle to address the problem.

Ok

> For drivers, I understand request_firmware()/request_firmware_nowait()
> and release_firmware() are enough. If many devices share one firmware,
> there is only one firmware kept in memory for their requests if one holds
> the firmware, and there is a refcount for it already, :-)
> 
> So I don't see why it is difficult to use request/release_firmware() inside
> drivers, :-)

The big problem can be summed up in one word "asynchronous". Having
either an automated handler for it before ->probe is called or having the
driver author cut and paste in

	if (!dev_request_firmware(dev, blah))
		return -EPROBE_DEFER;

avoids the need to deal with async completion after probe (and the
*horrible* case of 

	probe
		request firmware
	remove
	
	firmware ready
)

in each driver

Having an auto unload for it at the end is just neatness. Perhaps in fact
it should be devm_request_firmware() and use the mechanism we have ?
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