linux-kernel - Re: [PACTH v1] cdc-wdm: Clear read pipeline in case of error

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Message-ID: <1470220742.4612.7.camel@suse.com>
Date:	Wed, 03 Aug 2016 12:39:02 +0200
From:	Oliver Neukum <oneukum@...e.com>
To:	Robert Foss <robert.foss@...labora.com>
Cc:	pprabhu@...omium.org, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PACTH v1] cdc-wdm: Clear read pipeline in case of error

On Tue, 2016-08-02 at 10:37 -0400, Robert Foss wrote:
> 
> On 2016-08-02 09:59 AM, Oliver Neukum wrote:
> > On Tue, 2016-08-02 at 09:54 -0400, Robert Foss wrote:
> >>
> >> On 2016-08-02 08:23 AM, Oliver Neukum wrote:
> >>> On Thu, 2016-07-28 at 14:19 -0400, robert.foss@...labora.com wrote:
> >>>> From: Prathmesh Prabhu <pprabhu@...omium.org>
> >>>>
> >>>> Implemented queued response handling. This queue is processed every
> >>>> time the
> >>>> WDM_READ flag is cleared.
> >>>>
> >>>> In case of a read error, userspace may not actually read the data,
> >>>> since the
> >>>> driver returns an error through wdm_poll. After this, the underlying
> >>>> device may
> >>>> attempt to send us more data, but the queue is not processed. While
> >>>> userspace is
> >>>> also blocked, because the read error is never cleared.
> >>>
> >>> Could you explain why user space cannot just read more data?
> >>> That will clear the error.
> >>
> >> Userspace certainly could read more data, but for the case when
> >> userspace doesn't read and clear a potential an error, we still would
> >> like to not be stuck if the device sends more data. space
> >>
> >> I hope that answers your question, if not I'll try to be more elaborate.
> >
> > Clear, but why does that require the suppression of an error condition?
> > errors should always be delivered.
> 
> The goal is not to clear the error condition, but that is required to 
> not stay stuck.

How can that depend on what we return to user space?
In the driver we can continue just ignoring errors.
Now, if user space stops reading because we reported an error,
that is the decision user space has made. We cannot ignore errors
in the kernel because we don't like what user space does when it
sees the error.

	Regards
		Oliver