RE: [RFCv1 1/1] USB: EHCI: Do not return -EPIPE when hub is disconnected

From: "Erkka Talvitie" <erkka.talvitie@vincit.fi>
To: "'Alan Stern'" <stern@rowland.harvard.edu>
Cc: <gregkh@linuxfoundation.org>, <linux-usb@vger.kernel.org>,
	<claus.baumgartner@med.ge.com>
Subject: RE: [RFCv1 1/1] USB: EHCI: Do not return -EPIPE when hub is disconnected
Date: Thu, 5 Dec 2019 17:00:11 +0200	[thread overview]
Message-ID: <022001d5ab7c$b1e27380$15a75a80$@vincit.fi> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1912050929350.14919-100000@netrider.rowland.org>

> -----Original Message-----
> From: Alan Stern <stern@rowland.harvard.edu>
> Sent: torstai 5. joulukuuta 2019 16.38
> To: Erkka Talvitie <erkka.talvitie@vincit.fi>
> Cc: gregkh@linuxfoundation.org; linux-usb@vger.kernel.org;
> claus.baumgartner@med.ge.com
> Subject: RE: [RFCv1 1/1] USB: EHCI: Do not return -EPIPE when hub is
> disconnected
> 
> On Thu, 5 Dec 2019, Erkka Talvitie wrote:
> 
> > I tested this change and the issue did not reproduce.
> >
> > However when I was writing the comments for the patch I started to
> > think what happens in this following scenario:
> >
> > The PID Code is IN.
> >
> > 1. First there will be XACT, the CERR is decremented, let's say from 3
> > to 2 and the host controller executes a retry.
> > 2. On this next try there will happen the condition mentioned in the
> > Table
> > 4-13 of the EHCI specification so that the MMF is set and the queue is
> > halted (because it is IN).
> > 3. To my understanding now the execution enters to this first stall
> > check if, as CERR is > 0 and CERR < EHCI_TUNE_CERR.
> > 4. The -EPIPE (stall) is returned when actually the queue was halted
> > due to MMF - not stall - and the -EPROTO should be returned.
> >
> > Is my logic correct or am I missing something?
> 
> The same thought had occurred to me.
> 
> > If you agree with me then I would like to present you a bit more bold
> > (in a sense of amount of refactoring) RFC. In high level this another
> > RFC separates 1. error check and 2. stall check. For me this approach
> > is a bit easier to understand from the code. Or then please  propose
> > another solution.
> 
> I was going to suggest: Just check for MMF and PID == IN before checking
for
> STALL.  Everything else can remain the way it is.

Ok, just to double check:

I take the existing driver code (I will not apply the earlier RFC on top of
that) and apply one more check before the original stall check (that is):
} else if (QTD_CERR(token)) {

The check that I will add is checking MMF bit && PID == IN, and this check
comes right after the babble check, right?

Good, seems like a simple change. Yet I still prefer to test the change.
Unfortunately that goes to the next week as we have a national holiday
tomorrow. 
I will get back to you most likely on Monday.

> 
> Alan Stern
> 
Erkka Talvitie