Re: [PATCH v2 3/7] usb: xhci: Check for blocked disconnection

[Thinh Nguyen <Thinh.Nguyen@synopsys.com>]

Mathias Nyman wrote:
> On 28.4.2021 1.30, Thinh Nguyen wrote:
>>> On 10.4.2021 3.47, Thinh Nguyen wrote:
>>>> If there is a device with active enhanced super-speed (eSS) isoc IN
>>>> endpoint(s) behind one or more eSS hubs, DWC_usb31 (v1.90a and prior)
>>>> host controller will not detect the device disconnection until no more
>>>> isoc URB is submitted. If there's a device disconnection, internally
>>>> the wait for tHostTransactionTimeout (USB 3.2 spec 8.13) blocks the
>>>> other endpoints from being scheduled. So, it blocks the interrupt
>>>> endpoint of the eSS hub indicating the port change status.
>>>>
>>>> This can be an issue for applications that continuously submitting isoc
>>>> URBs to the xHCI. To work around this, stop processing new URBs after 3
>>>> consecutive isoc transaction errors. If new isoc transfers are queued
>>>> after the device is disconnected, the host will respond with USB
>>>> transaction error. After 3 consecutive USB transaction errors, the
>>>> driver can wait a period of time (at least 2 * largest periodic interval
>>>> of the topology) without ringing isoc endpoint doorbell to detect the
>>>> port change status. If there is no disconnection detected, ring the
>>>> endpoint doorbell to resume isoc transfers.
>>>
>>> Is that enough? many Isoc URBs queue 16 - 32 Isoc TRBs per URB.
>>> And drivers like UVC queue several URBs in advance.
>>
>> That's fine as long as the driver stops ringing more doorbell for a
>> certain period of time creating a gap that's enough to get the
>> notification from the interrupt endpoint. We tested with 128 isoc URBs
>> and was able to detect a disconnect after this delay.
> 
> Ok, if not ringing the endpoint is enough then that is better than
> stopping the whole endpoint. 
> 
>>>> This workaround tracks the max eSS periodic interval every time there's
>>>> an endpoint added or dropped, which happens when there's bandwidth
>>>> check. So, scan the topology and update the xhci->max_ess_interval
>>>> whenever there's a bandwidth check. Introduced a new flag
>>>> VDEV_DISCONN_CHECK_PENDING to prevent ringing the doorbell while waiting
>>>> for a disconnection status. After 2 * max_ess_interval time and no
>>>> disconnection detected, a delayed work will ring the doorbell to resume
>>>> the active isoc transfers.
>>>
>>> Sounds very elaborate for a vendor specific disconnect workaround.
>>> Isn't there a simpler way?
>>>
>>> Maybe stop all isoc in endpoints if one them has 3 consecutive transaction error,
>>> wait for 2x hub interrupt interval time, and then restart the endpoints if there is
>>> no disconnect?
>>
>> We can also do this (but without stop + restart the endpoints). It just
>> creates a slightly larger gap that may be more noticeable to the user if
>> there's no actual disconnection.
> 
> Ok, if blocking the doorbell is enough then it sounds better.
> 
> How about that max interval tracking, is it necessary?
> It will block the doorbell from 250us up to several seconds.
> Is there some reasonable single value that can be used instead?

Yeah, I agree with you. I think we can find a number that satisfies with
most applications while keeping this change small and less intrusive.

> 
>>>
>>> There is bigger concern with this series, it scatters a lot of vendor specific code 
>>> around the generic xhci driver. It's not very clear afterwards what code is part of the
>>> workaround and what is generic code.
>>>
>>> We just got a lot of the Mediatek code moved to xhci-mtk*, maybe its time to add xhci-snps.c
>>> instead of using the generic platform driver with tons of workarounds and quirks.
>>>
>>
>> Thanks for the reviews. I need to look into how this can be done. May
>> need your suggestion as not every scenarios can be overridden
>> easily/cleanly.
> 
> true, no easy overrides for this transfer error / doorbell blocking workaround.
> Needs a bit of work
> 

Thanks,
Thinh