linux-kernel - Re: [PATCH v11] i2c: virtio: add a virtio i2c frontend driver

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Date:   Fri, 2 Jul 2021 11:36:45 +0800
From:   Jie Deng <jie.deng@...el.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
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 v11] i2c: virtio: add a virtio i2c frontend driver


On 2021/7/1 14:18, Viresh Kumar wrote:
> On 01-07-21, 14:10, Jie Deng wrote:
>> I think a fixed number of sgs will make things easier to develop backend.
> Yeah, but it looks awkward to send a message buffer which isn't used
> at all. From protocol's point of view, it just looks wrong/buggy.
>
> The backend can just look at the number of elements received, they
> can either be 2 (in case of zero-length) transfer, or 3 (for
> read/write) and any other number is invalid.
>

OK. Let's add the following two lines to make sure that msg_buf is only
sent when the msgs len is not zero. And backend judges whether it is
a zero-length request by checking the number of elements received.

  + 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;
+}