linux-kernel - Re: [RESEND][PATCH] Bluetooth: Make request workqueue freezable

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Message-ID: <Pine.LNX.4.44L0.1505191314290.1453-100000@iolanthe.rowland.org>
Date:	Tue, 19 May 2015 13:17:57 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Marcel Holtmann <marcel@...tmann.org>
cc:	Takashi Iwai <tiwai@...e.de>, Laura Abbott <labbott@...hat.com>,
	Laura Abbott <labbott@...oraproject.org>,
	"Gustavo F. Padovan" <gustavo@...ovan.org>,
	Johan Hedberg <johan.hedberg@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	"bluez mailin list (linux-bluetooth@...r.kernel.org)" 
	<linux-bluetooth@...r.kernel.org>, netdev <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ming Lei <ming.lei@...onical.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	<linux-usb@...r.kernel.org>
Subject: Re: [RESEND][PATCH] Bluetooth: Make request workqueue freezable

On Tue, 19 May 2015, Marcel Holtmann wrote:

> Hi Alan,
> 
> >>>> I am not convinced. Now we are hacking the Bluetooth core layer
> >>>> (which has nothing to do with the drivers suspend/resume or
> >>>> probe) to do something different so that we do not see this
> >>>> warning.
> >>>> 
> >>>> I can not do anything about the platform in question choosing a
> >>>> unplug/replug for suspend/resume instead of having a proper USB
> >>>> suspend and resume handling. That is pretty much out of our
> >>>> control.
> > 
> > Actually one can do something about this.  I mean, one _can_ implement
> > proper USB suspend and resume handling in the Bluetooth driver.  At
> > this point the details aren't clear to me, but perhaps if the driver in
> > question had a reset_resume callback then it might work better.
> 
> the btusb.ko driver has suspend/resume support. Are you saying we
> also need reset_resume support?

I don't know; I'm not familiar enough with how Bluetooth works.  If the 
device loses power and requires its firmware to be loaded again, then a 
reset_resume would end up doing much the same thing as probe anyway.  
So implementing reset_resume might not make much difference.

> >>>> I would rather have the USB subsystem delay the probe()
> >>>> callback if we tell it to.
> > 
> > This is possible.  I am not sure it would be the right thing to do,
> > though.  What happens if the probe routine gets called early on during
> > the boot-up procedure, before userspace is up and running?  The same
> > thing should happen here.
> 
> For modules this will be hard. Since you need userspace before being
> able to load the modules. If built-in code, then in theory this might
> be possible. Depending on the order of the init sections.

Yes, I meant built-in.

> >>>> Of just have request_firmware()
> >>>> actually sleep until userspace is ready. Seriously, why is
> >>>> request_firmware not just sleeping for us.
> > 
> > It won't work.  The request_firmware call is part of the probe 
> > sequence, which in turn is part of the resume sequence.  Userspace 
> > doesn't start running again until the resume sequence is finished.  If 
> > request_firmware waited for userspace, it would hang.
> 
> Then I really have no idea on how to solve this unless we silence the
> warning from request_firmware. From a driver perspective we go back
> trough probe(). So the driver has to treat this as a new device.

Oliver's suggestion to keep the firmware in memory may be the only
reasonable solution.

Alan Stern

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