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Subject: Re: [PATCH v10] i2c: virtio: add a virtio i2c frontend driver
To:     Viresh Kumar <viresh.kumar@linaro.org>
Cc:     linux-i2c@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, mst@redhat.com, wsa@kernel.org,
        jasowang@redhat.com, wsa+renesas@sang-engineering.com,
        andriy.shevchenko@linux.intel.com, conghui.chen@intel.com,
        arnd@arndb.de, kblaiech@mellanox.com,
        jarkko.nikula@linux.intel.com, Sergey.Semin@baikalelectronics.ru,
        rppt@kernel.org, loic.poulain@linaro.org, tali.perry1@gmail.com,
        u.kleine-koenig@pengutronix.de, bjorn.andersson@linaro.org,
        yu1.wang@intel.com, shuo.a.liu@intel.com, stefanha@redhat.com,
        pbonzini@redhat.com
References: <226a8d5663b7bb6f5d06ede7701eedb18d1bafa1.1616493817.git.jie.deng@intel.com>
 <20210323090108.ygx76exdgzudeeqi@vireshk-i7>
 <20210323093839.n7cq7f5poebqdwit@vireshk-i7>
 <5a415dbe-8e3b-2731-cc52-19aeadda1a17@intel.com>
 <20210324035225.skkllxexjl65gs6x@vireshk-i7>
From:   Jie Deng <jie.deng@intel.com>
Message-ID: <70a2d0f7-ac48-b477-8368-7ef55696bff1@intel.com>
Date:   Wed, 24 Mar 2021 12:00:33 +0800
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On 2021/3/24 11:52, Viresh Kumar wrote:
> 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 ?"
>   


It can't start until the virtqueue_kick() is called. Call virtqueue_kick 
then wait.