linux-kernel - Re: [PATCH v10] i2c: virtio: add a virtio i2c frontend driver

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Message-ID: <20210324035225.skkllxexjl65gs6x@vireshk-i7>
Date:   Wed, 24 Mar 2021 09:22:25 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Jie Deng <jie.deng@...el.com>
Cc:     linux-i2c@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, mst@...hat.com, wsa@...nel.org,
        jasowang@...hat.com, wsa+renesas@...g-engineering.com,
        andriy.shevchenko@...ux.intel.com, conghui.chen@...el.com,
        arnd@...db.de, kblaiech@...lanox.com,
        jarkko.nikula@...ux.intel.com, Sergey.Semin@...kalelectronics.ru,
        rppt@...nel.org, loic.poulain@...aro.org, tali.perry1@...il.com,
        u.kleine-koenig@...gutronix.de, bjorn.andersson@...aro.org,
        yu1.wang@...el.com, shuo.a.liu@...el.com, stefanha@...hat.com,
        pbonzini@...hat.com
Subject: Re: [PATCH v10] i2c: virtio: add a virtio i2c frontend driver

On 24-03-21, 08:53, Jie Deng wrote:
> 
> On 2021/3/23 17:38, Viresh Kumar wrote:
> > On 23-03-21, 14:31, Viresh Kumar wrote:
> > > On 23-03-21, 22:19, Jie Deng wrote:
> > > > +static int virtio_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
> > > > +{
> > > > +	struct virtio_i2c *vi = i2c_get_adapdata(adap);
> > > > +	struct virtqueue *vq = vi->vq;
> > > > +	struct virtio_i2c_req *reqs;
> > > > +	unsigned long time_left;
> > > > +	int ret, nr;
> > > > +
> > > > +	reqs = kcalloc(num, sizeof(*reqs), GFP_KERNEL);
> > > > +	if (!reqs)
> > > > +		return -ENOMEM;
> > > > +
> > > > +	mutex_lock(&vi->lock);
> > > > +
> > > > +	ret = virtio_i2c_send_reqs(vq, reqs, msgs, num);
> > > > +	if (ret == 0)
> > > > +		goto err_unlock_free;
> > > > +
> > > > +	nr = ret;
> > > > +	reinit_completion(&vi->completion);
> > > I think I may have found a possible bug here. This reinit_completion() must
> > > happen before we call virtio_i2c_send_reqs(). It is certainly possible (surely
> > > in corner cases) that virtio_i2c_msg_done() may get called right after
> > > virtio_i2c_send_reqs() and before we were able to call reinit_completion(). And
> > > in that case we will never see the completion happen at all.
> > > 
> > > > +	virtqueue_kick(vq);
> > I may have misread this. Can the actually start before virtqueue_kick() is
> > called ?

I didn't write it properly here. I wanted to say,

"Can the _transfer_ actually start before virtqueue_kick() is called ?"
 
> No. It starts when wait_for_completion_timeout is called.

No, the transfer doesn't have anything to do with wait_for_completion_timeout().
And if complete() gets called before wait_for_completion_timeout() is called,
then wait_for_completion_timeout() will simply return back.

> So it should be fine here.
> 
> 
> >   If not, then completion may be fine where it is.
> > 

-- 
viresh