Re: Options for forcing dwc3 gadget to only accept superspeed

[Felipe Balbi <balbi@kernel.org>]

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Hi,

Claus Stovgaard <claus.stovgaard@gmail.com> writes:
> On tir, 2020-05-12 at 15:52 -0400, Alan Stern wrote:
>> On Tue, May 12, 2020 at 09:25:38PM +0200, Claus Stovgaard wrote:
>> > 
>> > In a certain scenario I would like to force the dwc3 to only
>> > connect
>> > via superspeed and not fall back to USB2.
>> > 
>> > What options exist for forcing the dwc3 to keep retry?
>> 
>> The USB-3 spec forbids devices from operating only at SuperSpeed.  
>> Devices must be able to connect at high speed, although possibly
>> with 
>> reduced functionality.
>> 
>> Alan Stern
>> 
>
> I understand the requirement from the USB 3 specification. Though in
> the scenario for this specific device, it is not about comply with the
> USB 3 specification, but my question is rather what options I have for
> not comply with the specification here, and then force retry of USB 3,
> using the dwc3 as device.
>
> The device is in a fixed mounting with a fixed host. Sometimes when the
> host and device is powered up, it ends in high-speed instead of super-
> speed. I would like the option for "I will not be compliant with USB,
> but rather retry super-speed".

If it's "ending in super-speed" this means it tried RX.Detect and it
failed, thus falling back to high-speed. It's likely that the signal
quality in your setup has degraded or is, at least, sub-par.

Forcing a SS-only setup would just give you a device that doesn't even
enumerate in some cases.

Could you capture dwc3 tracepoints of the problem?

Also, which kernel version are you running? Which platform?

best

-- 
balbi

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