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

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Date:   Wed, 24 Mar 2021 09:50:46 +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 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);
> +	virtqueue_kick(vq);

Coming back to this again, what is the expectation from the other side for this
? I mean there is no obvious relation between the *msgs* which we are going to
transfer (from the other side's or host's point of view). When should the host
OS call its virtqueue_kick() counterpart ?

Lemme give an example for this. Lets say that we need to transfer 3 messages
here in this routine. What we did was we prepared virtqueue for all 3 messages
together and then called virtqueue_kick().

Now if the other side (host) processes the first message and sends its reply
(with virtqueue_kick() counterpart) before processing the other two messages,
then it will end up calling virtio_i2c_msg_done() here. That will make us call
virtio_i2c_complete_reqs(), while only the first messages is processed until
now and so we will fail for the other two messages straight away.

Should we send only 1 message from i2c-virtio linux driver and then wait for
virtio_i2c_msg_done() to be called, before sending the next message to make sure
it doesn't break ?

-- 
viresh