netdev - Re: [PATCH 1/2] usbnet: allow status interrupt URB to always be active

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Date:	Mon, 07 Jan 2013 09:21:30 -0600
From:	Dan Williams <dcbw@...hat.com>
To:	Bjørn Mork <bjorn@...k.no>
Cc:	Oliver Neukum <oliver@...kum.org>,
	Elina Pasheva <epasheva@...rrawireless.com>,
	netdev@...r.kernel.org, linux-usb@...r.kernel.org,
	Rory Filer <rfiler@...rrawireless.com>,
	Phil Sutter <phil@....cc>
Subject: Re: [PATCH 1/2] usbnet: allow status interrupt URB to always be
 active

On Sat, 2013-01-05 at 11:59 +0100, Bjørn Mork wrote:
> Dan Williams <dcbw@...hat.com> writes:
> > On Fri, 2013-01-04 at 23:16 +0100, Oliver Neukum wrote:
> >> On Friday 04 January 2013 10:48:16 Dan Williams wrote:
> >> > Some drivers (ex sierra_net) need the status interrupt URB
> >> > active even when the device is closed, because they receive
> >> > custom indications from firmware.  Allow sub-drivers to set
> >> > a flag that submits the status interrupt URB on probe and
> >> > keeps the URB alive over device open/close.  The URB is still
> >> > killed/re-submitted for suspend/resume, as before.
> >> > 
> >> > Signed-off-by: Dan Williams <dcbw@...hat.com>
> >> > ---
> >> > Oliver: alternatively, is there a problem with *always*
> >> > submitting the interrupt URB, and then simply not calling
> >> > the subdriver's .status function when the netdev is
> >> > closed?  That would be a much simpler patch.
> >> 
> >> That is quite radical. We have no idea what a device
> >> does when we do not react to a status update. I would
> >> much prefer to not take the risk.
> >> Besides, we don't use bandwidth if we don't have to.
> >
> > Ok, so scratch the alternative.  Thus, does the posted patch look like
> > the right course of action?
> >
> > If I wasn't clear enough before, sierra_net needs to listen to the
> > status interrupt URB to receive the custom Restart indication as part of
> > the driver's device setup.  Thus for sierra_net at least, tying the
> > status interrupt URB submission to device open/close isn't right.
> >
> > I'd previously done a patch to handle this all in sierra_net, but the
> > problem there is suspend/resume: without directly accessing the usbnet
> > structure's ->suspend_count member (icky!) sierra_net can't correctly
> > kill/submit the URB itself.  So I went with a flag to usbnet that Sierra
> > can set.
> 
> Just a few random thoughts...
> 
> The sierra_net driver would have been an excellent candidate for the
> cdc-wdm subdriver model if that had existed when sierra_net was written.
> I assume the current driver only implements a minimal subset of the
> DirectIP HIP protocol embedded in CDC, and exporting this to userspace
> instead would have made it possible to extend that support without
> changing the driver, in addtion to making the driver much more robust
> against firmware differences.  It would have eliminated the problem you
> are facing and removed the minidriver workqueue complexity.
> 
> But I guess it's too late to change this now.  In theory we could
> implement the cdc-wdm hooks that were initially proposed for cdc_mbim
> and rewrite sierra_net to use it while being backwards compatible.
> Don't think anyone wants to do either, so forget about it...
> 
> You can still use a trick similar to what qmi_wwan and cdc_mbim does to
> take over the status endpoint from usbnet: By not implementing .status,
> and possibly setting dev->status to NULL in .bind, you are free to
> handle the status endpoint entirely inside the minidriver.  Not sure if
> that is smart though.  You would have to reimplement init_status and
> intr_complete from usbnet, and kill or resubmit the interrupt urb on
> suspend/resume/disconnect yourself.

That was exactly the approach of my first patch.  But that was icky
because we need to also track suspend/resume state by looking at
internal members of usbnet's structure.  Yes, it's specific to Sierra,
but it's pretty much a layer violation.  So I went with the flag
approach.

Dan

> The new usbnet flag is probably a better solution.
> 
> FWIW, I agree with Oliver that always submitting the interrupt URB is
> both risky and will cause too much unnecessary USB activity for most
> usbnet devices.
> 
> 
> Bjørn
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