linux-kernel - Re: [PATCH v14] i2c: virtio: add a virtio i2c frontend driver

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Date:   Fri, 9 Jul 2021 09:14:07 +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, gregkh@...uxfoundation.org,
        vincent.guittot@...aro.org, alex.bennee@...aro.org
Subject: Re: [PATCH v14] i2c: virtio: add a virtio i2c frontend driver

On 09-07-21, 10:25, Jie Deng wrote:
> Add an I2C bus driver for virtio para-virtualization.
> 
> The controller can be emulated by the backend driver in
> any device model software by following the virtio protocol.
> 
> The device specification can be found on
> https://lists.oasis-open.org/archives/virtio-comment/202101/msg00008.html.
> 
> By following the specification, people may implement different
> backend drivers to emulate different controllers according to
> their needs.
> 
> Co-developed-by: Conghui Chen <conghui.chen@...el.com>
> Signed-off-by: Conghui Chen <conghui.chen@...el.com>
> Signed-off-by: Jie Deng <jie.deng@...el.com>
> ---
> Changes v13 -> v14
> 	- Put the headers in virtio_i2c.h in alphabetical order.
> 	- Dropped I2C_FUNC_SMBUS_QUICK support.
> 	- Dropped few unnecessary variables and checks.
> 	- Use "num" everywhere instead of num or nr, to be consistent.
> 	- Added few comments which make the design more clear. 

Thanks a lot for following this up so far :)

> +static int virtio_i2c_prepare_reqs(struct virtqueue *vq,
> +				   struct virtio_i2c_req *reqs,
> +				   struct i2c_msg *msgs, int num)
> +{
> +	struct scatterlist *sgs[3], out_hdr, msg_buf, in_hdr;
> +	int i;
> +
> +	for (i = 0; i < num; i++) {
> +		int outcnt = 0, incnt = 0;
> +
> +		/*
> +		 * We don't support 0 length messages and so masked out
> +		 * I2C_FUNC_SMBUS_QUICK in virtio_i2c_func().
> +		 */
> +		if (!msgs[i].len)
> +			break;
> +
> +		/*
> +		 * 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 != num - 1)
> +			reqs[i].out_hdr.flags = cpu_to_le32(VIRTIO_I2C_FLAGS_FAIL_NEXT);
> +
> +		sg_init_one(&out_hdr, &reqs[i].out_hdr, sizeof(reqs[i].out_hdr));
> +		sgs[outcnt++] = &out_hdr;
> +
> +		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;
> +
> +		if (virtqueue_add_sgs(vq, sgs, outcnt, incnt, &reqs[i], GFP_KERNEL)) {
> +			i2c_put_dma_safe_msg_buf(reqs[i].buf, &msgs[i], false);
> +			break;
> +		}
> +	}
> +
> +	return i;
> +}

Wolfram, in case you wonder why we don't error out early as discussed earlier,
then ...

> +static int virtio_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg *msgs,
> +			   int num)
> +{

...

> +	/*
> +	 * For the case where count < num, i.e. we weren't able to queue all the
> +	 * msgs, ideally we should abort right away and return early, but some
> +	 * of the messages are already sent to the remote I2C controller and the
> +	 * virtqueue will be left in undefined state in that case. We kick the
> +	 * remote here to clear the virtqueue, so we can try another set of
> +	 * messages later on.
> +	 */

... here is the reasoning for that.

Please see if you can still get it merged into 5.14-rc1/2. Thanks.

Reviewed-by: Viresh Kumar <viresh.kumar@...aro.org>
Tested-by: Viresh Kumar <viresh.kumar@...aro.org>

-- 
viresh