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Subject: Re: [PATCH v11] 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,
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        yu1.wang@intel.com, shuo.a.liu@intel.com, conghui.chen@intel.com,
        stefanha@redhat.com
References: <510c876952efa693339ab0d6cc78ba7be9ef6897.1625104206.git.jie.deng@intel.com>
 <20210701040436.p7kega6rzeqz5tlm@vireshk-i7>
 <cb35472d-f79e-f3f8-405f-35c699d897a1@intel.com>
 <20210701061846.7u4zorimzpmb66v7@vireshk-i7>
From:   Jie Deng <jie.deng@intel.com>
Message-ID: <34092cb2-03f9-231d-8769-4e45ed51c30f@intel.com>
Date:   Fri, 2 Jul 2021 11:36:45 +0800
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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;
+}