linux-kernel - Re: [PATCH v12] i2c: virtio: add a virtio i2c frontend driver

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Date:   Mon, 5 Jul 2021 08:10:56 +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, wsa@...nel.org,
        wsa+renesas@...g-engineering.com, mst@...hat.com, arnd@...db.de,
        jasowang@...hat.com, andriy.shevchenko@...ux.intel.com,
        yu1.wang@...el.com, shuo.a.liu@...el.com, conghui.chen@...el.com,
        stefanha@...hat.com
Subject: Re: [PATCH v12] i2c: virtio: add a virtio i2c frontend driver

I think we missed the first deadline for 5.14-rc1 as Wolfram should be out of
office now. Anyway lets make sure we fix all the pending bits before he is back
and see if we can still pull it off by rc2.

On 02-07-21, 16:46, Jie Deng wrote:
> diff --git a/drivers/i2c/busses/i2c-virtio.c b/drivers/i2c/busses/i2c-virtio.c
> +static int virtio_i2c_send_reqs(struct virtqueue *vq,

It would be better to rename this to virtio_i2c_prepare_reqs instead, as this
doesn't send anything.

> +				struct virtio_i2c_req *reqs,
> +				struct i2c_msg *msgs, int nr)
> +{
> +	struct scatterlist *sgs[3], out_hdr, msg_buf, in_hdr;
> +	int i, outcnt, incnt, err = 0;
> +
> +	for (i = 0; i < nr; i++) {
> +		/*
> +		 * Only 7-bit mode supported for this moment. For the address format,
> +		 * Please check the Virtio I2C Specification.
> +		 */
> +		reqs[i].out_hdr.addr = cpu_to_le16(msgs[i].addr << 1);
> +
> +		if (i != nr - 1)
> +			reqs[i].out_hdr.flags = cpu_to_le32(VIRTIO_I2C_FLAGS_FAIL_NEXT);
> +
> +		outcnt = incnt = 0;
> +		sg_init_one(&out_hdr, &reqs[i].out_hdr, sizeof(reqs[i].out_hdr));
> +		sgs[outcnt++] = &out_hdr;
> +
> +		if (msgs[i].len) {
> +			reqs[i].buf = i2c_get_dma_safe_msg_buf(&msgs[i], 1);
> +			if (!reqs[i].buf)
> +				break;
> +
> +			sg_init_one(&msg_buf, reqs[i].buf, msgs[i].len);
> +
> +			if (msgs[i].flags & I2C_M_RD)
> +				sgs[outcnt + incnt++] = &msg_buf;
> +			else
> +				sgs[outcnt++] = &msg_buf;
> +		}
> +
> +		sg_init_one(&in_hdr, &reqs[i].in_hdr, sizeof(reqs[i].in_hdr));
> +		sgs[outcnt + incnt++] = &in_hdr;
> +
> +		err = virtqueue_add_sgs(vq, sgs, outcnt, incnt, &reqs[i], GFP_KERNEL);
> +		if (err < 0) {
> +			i2c_put_dma_safe_msg_buf(reqs[i].buf, &msgs[i], false);
> +			break;
> +		}
> +	}
> +
> +	return i;
> +}

The right way of doing this is is making this function return - Error on failure
and 0 on success. There is no point returning number of successful additions
here.

Moreover, on failures this needs to clean up (free the dmabufs) itself, just
like you did i2c_put_dma_safe_msg_buf() at the end. The caller shouldn't be
required to handle the error cases by freeing up resources.

> +static int virtio_i2c_complete_reqs(struct virtqueue *vq,
> +				    struct virtio_i2c_req *reqs,
> +				    struct i2c_msg *msgs, int nr,
> +				    bool fail)
> +{
> +	struct virtio_i2c_req *req;
> +	bool failed = fail;
> +	unsigned int len;
> +	int i, j = 0;
> +
> +	for (i = 0; i < nr; i++) {
> +		/* Detach the ith request from the vq */
> +		req = virtqueue_get_buf(vq, &len);
> +
> +		/*
> +		 * Condition (req && req == &reqs[i]) should always meet since
> +		 * we have total nr requests in the vq.
> +		 */
> +		if (!failed && (WARN_ON(!(req && req == &reqs[i])) ||
> +		    (req->in_hdr.status != VIRTIO_I2C_MSG_OK)))

What about writing this as:

		if (!failed && (WARN_ON(req != &reqs[i]) ||
		    (req->in_hdr.status != VIRTIO_I2C_MSG_OK)))

We don't need to check req here since if req is NULL, we will not do req->in_hdr
at all.

> +			failed = true;
> +
> +		i2c_put_dma_safe_msg_buf(reqs[i].buf, &msgs[i], !failed);
> +		if (!failed)
> +			++j;
> +	}
> +
> +	return (fail ? -ETIMEDOUT : j);
> +}
> +

-- 
viresh