linux-kernel - Re: [PATCH v12 7/8] usb: Adding SuperSpeed support to dummy

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Message-Id: <BD27AEAC-1E6D-480C-9E18-A970B992AD75@ti.com>
Date:	Tue, 24 May 2011 00:06:29 +0300
From:	Felipe Balbi <balbi@...com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Felipe Balbi <balbi@...com>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	Tanya Brokhman <tlinder@...eaurora.org>, <greg@...ah.com>,
	<linux-usb@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>,
	<ablay@...eaurora.org>,
	"'open list'" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v12 7/8] usb: Adding SuperSpeed support to dummy_hcd

Hi,

(replying from another PC, not sure if formatting
will be broken, hopefully not)

On May 23, 2011, at 7:28 PM, Alan Stern wrote:
>>> Since in dummy_hcd all of this is much simpler I think that the device speed
>>> should be determined by driver->speed and "which type of cable the
>>> connection was made over - SS or HS". The "cable type" is exactly what the
>>> module parameter is.
>> 
>> I really don't understand this.  You're going to have a module parameter
>> for what type of cable is plugged in?
> 
> It would be more accurate to say the module parameter will be used to
> force the connection to run at a lower speed than the maximum possible.  
> This is kind of like what happens when you plug in a SuperSpeed device 
> using a USB-2 cable -- the connection runs at a lower speed than it 
> could have.

if it's something like that, for sure we can have the module parameter. But
plugging a USB2 gadget/function to a USB3-capable UDC/HCD should
work fine.

With Tatyana's patches, if we load a USB2 g_zero to dummy_hcd, enumeration
will fail where it shouldn't. This has been my whole point ;-) Maybe I wasn't
clear enough.

>> What about the case where SuperSpeed enumeration
>> fails and you have to fall back to high speed?
> 
> If SuperSpeed enumeration fails, say because the device doesn't have 
> any SuperSpeed descriptors, xhci-hcd doesn't fall back to high speed, 
> does it?  dummy-hcd should behave the same way.

it should at least. Isn't that what happens between EHCI/OHCI ? HS Chirp
sequencing fails, then we fall back to FullSpeed.

>> It seems like you really
>> need to handle both speeds and the speed fall back parameter in the same
>> driver.  Isn't there some other gadget driver that has a fall back to
>> full or low speed when high speed enumeration fails?
> 
> That's a property of the gadget driver, not the UDC driver.  dummy-hcd 
> is a UDC driver (and an HCD too).

USB3.0 dummy_hcd should still enumerate USB2.0 gadget drivers.

-- 
balbi

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