linux-kernel - Re: [V2 PATCH 01/10] added media agnostic (MA) USB HCD driver

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Date:	Wed, 12 Nov 2014 17:03:18 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Sean O. Stalley" <sean.stalley@...el.com>
cc:	Stephanie Wallick <stephanie.s.wallick@...el.com>,
	<linux-kernel@...r.kernel.org>, <gregkh@...uxfoundation.org>,
	<linux-usb@...r.kernel.org>, <devel@...verdev.osuosl.org>
Subject: Re: [V2 PATCH 01/10] added media agnostic (MA) USB HCD driver

On Wed, 12 Nov 2014, Sean O. Stalley wrote:

> On Tue, Nov 11, 2014 at 10:54:30AM -0500, Alan Stern wrote:
> > On Mon, 10 Nov 2014, Stephanie Wallick wrote:
> > 
> > > +static struct mausb_hcd mhcd;
> > 
> > Only one statically-allocated structure?  What if somebody wants to 
> > have more than one of these things in their system?
> > 
> 
> Our plan to support multiple MA devices is to have them all connected
> to the same virtual host controller, so only 1 would be needed.
> 
> Would you prefer we have 1 host controller instance per MA device?
> We are definitely open to suggestions on how this should be architected.

I haven't read the MA USB spec, so I don't know how it's intended to 
work.  Still, what happens if you create a virtual host controller 
with, say, 16 ports, and then someone wants to connect a 17th MA 
device?

Also, I noticed that your patch adds a new bus type for these MA host 
controllers.  It really seems like overkill to have a whole new bus 
type if there's only going to be one device on it.

> If we get rid of these locks, endpoints can't run simultaneously.
> MA USB IN endpoints have to copy data, which could take a while.

I don't know what you mean by "run simultaneously".  Certainly multiple 
network packets can be transmitted and received concurrently even if 
you use a single spinlock, since your locking won't affect the 
networking subsystem.

> Couldn't this cause a bottleneck?

Probably not enough to matter.  After all, the other host controller
drivers rely on a single spinlock.  And if it did matter, you could
drop the spinlock while copying the data.

Alan Stern

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