linux-kernel - Re: [PATCH v2 5/5] xhci: Correct handling of one-TRB isoc TD on Etron xHCI host

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Message-ID: <CAHN5xi2XLnp+VhM-6QxWeMvZ_uQRSqaeHSqEjab+rSNZ9E98pw@mail.gmail.com>
Date: Wed, 30 Oct 2024 13:17:51 +0800
From: Kuangyi Chiang <ki.chiang65@...il.com>
To: Michał Pecio <michal.pecio@...il.com>
Cc: gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org, 
	linux-usb@...r.kernel.org, mathias.nyman@...el.com
Subject: Re: [PATCH v2 5/5] xhci: Correct handling of one-TRB isoc TD on Etron
 xHCI host

Hi,

Thank you for the review.

Michał Pecio <michal.pecio@...il.com> 於 2024年10月28日 週一 下午5:54寫道：
>
> Hi,
>
> That's a bug I'm familiar with.
>
> > Unplugging a USB3.0 webcam while streaming results in errors
> > like this
>
> Not only unplugging but also any random error due to EMI or bad cable.
>
> > If an error is detected while processing an one-TRB isoc TD,
> > the Etron xHC generates two transfer events for the TRB that
> > the error was detected on. The first event is "USB Transcation
> > Error", and the second event is "Success".
>
> IIRC, it wasn't just Transaction Errors but any sort of error, like
> Babble or Bandwidth Overrun. But not sure about Missed Service, etc.
>
> And IIRC I confirmed that it was *not* the case on Short Packet.

Yes, it is not.

>
> Also, I'm 99% sure the problem is not limited to one-TRB TDs, but
> it occurs every time there is an error on the last TRB of any TD.

Yes, this can happen, I didn't account for this scenario.

>
> > As a solution, we can set the flag after the first error event
> > and don't print the error message after the second event if the
> > flag is set.
>
> Yes, but I think it would be better to use error_mid_td instead of
> last_td_was_short, so that the TD is only freed on the final event,
> not on the first one.
>
> The spec is clear that we should only free TRBs when the xHC is done
> with them. Maybe it wouldn't be a problem in this case, and it surely
> wouldn't be worse than what happens with Etron today, but IMO it could
> be a real (even if rare) problem in other cases when this flag is used,
> so I would rather remove the flag and handle short packets as per spec.

Thank you for the explanation and suggestion. Maybe I should start
trying to use error_mid_td to solve this problem.

>
> Regards,
> Michal

Thanks,
Kuangyi Chiang