Re: [PATCH v4 6/6] usb:cdns3 Fix for stuck packets in on-chip OUT buffer.

[Roger Quadros]

Hi,

On 21/02/2019 09:14, Felipe Balbi wrote:
> 
> Hi,
> 
> (please break your emails at 80-columns)
> 
> Pawel Laszczak <pawell@xxxxxxxxxxx> writes:
>>>> One more thing. Workaround has implemented algorithm that decide for which
>>>> endpoint it should be enabled.  e.g for composite device MSC+NCM+ACM it
>>>> should work only for ACM OUT endpoint.
>>>>
>>>
>>> If ACM driver didn't queue the request for ACM OUT endpoint, why does the
>>> controller accept the data at all?
>>>
>>> I didn't understand why we need a workaround for this. It should be standard
>>> behaviour to NAK data if function driver didn't request for all endpoints.
>>
>> Yes, I agree with you. Controller shouldnât accept such packet. As I know this
>> behavior will be fixed in RTL.
>>
>> But I assume that some older version of this controller are one the market,
>> and driver should work correct with them.
>>
>> In the feature this workaround can be limited only to selected controllers.
>>
>> Even now I assume that it can be enabled/disabled by module parameter.
> 
> no module parameters, please. Use revision detection in runtime.
> 

This is about whether to enable or disable the workaround.
By default we don't want this workaround to be enabled.

I'm debating whether we should have this workaround at all or not.

It has the following problems.

1) It ACKs packets even when gadget end is not ready to accept the transfers.
2) It stores these packets in a temporary buffer and then pushes them to the
gadget driver whenever the gadget driver is ready to process the data.
3) Since the gadget driver can become ready at an indefinite time in the
future, it poses 2 problems:
 a) It is sending stale data to the sink. (problematic at next protocol level?)
 b) If this temporary buffer runs out we still hit the lock up issue.

I think the right solution is to make sure that the gadget driver is always
reading all the enabled OUT endpoints *or* (keep the OUT endpoints disabled
if gadget driver is not ready to process OUT transfers).

cheers,
-roger
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