linux-kernel - Re: [PATCH 2/2] i2c: virtio: fix completion handling

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Date:   Fri, 29 Oct 2021 13:54:46 +0200
From:   Vincent Whitchurch <vincent.whitchurch@...s.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
CC:     Jie Deng <jie.deng@...el.com>, "wsa@...nel.org" <wsa@...nel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel <kernel@...s.com>
Subject: Re: [PATCH 2/2] i2c: virtio: fix completion handling

On Wed, Oct 20, 2021 at 12:47:09PM +0200, Viresh Kumar wrote:
> On 20-10-21, 12:38, Vincent Whitchurch wrote:
> > I don't quite understand how that would be safe since
> > virtqueue_add_sgs() can fail after a few iterations and all queued
> > request buffers can have FAIL_NEXT set.  In such a case, we would end up
> > waiting forever with your proposed change, wouldn't we?
> 
> Good point. I didn't think of that earlier.
> 
> I think a good simple way of handling this is counting the number of
> buffers sent and received. Once they match, we are done. That
> shouldn't break anything else I believe.

That could work, but it's not so straightforward since you would have to
introduce locking to prevent races since the final count is only known
after virtio_i2c_prepare_reqs() completes, while the callback could be
called before that.  Please do not hesitate to send out a patch to fix
it that way if that is what you prefer.