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Subject: Re: [virtio-comment] [PATCH v3] virtio-i2c: add the device specification

On 2020/10/29 4:10, Paolo Bonzini wrote:
On 27/10/20 07:00, Jie Deng wrote:
+The driver queues requests to the virtqueue, and they are used by the
+device. The request is the representation of one segment of an I2C
+transaction. Each request is of form:
+struct virtio_i2c_req {
+        le16 addr;
+        le16 flags;
+        le16 len;
+        u8 buf[];
+        u8 status;

one extremely common two-segment I2C transaction is not reproducible
with virtio-i2c.  This is when a write is followed by a read: the master
starts off the transmission with a write, then sends a second START,
then continues with a read from the same address.

Would it make sense to define the request like

struct virtio_i2c_req {
	le16 addr;
	le16 written;
	le16 read;
	u8 bufwrite[];
	u8 status;
	u8 bufread[];




Hi Paolo,

I'm not sure why this two-segment I2C transaction is not reproducible ?

my understanding is that we don't need to care about the START/STOP/RESTART conditions between virtio-i2c frontend and backend. These conditions only make sense when data is exchanged between master and slave devices, rather than virtio frontend and backend.

Only the backend may need to consider these conditions when doing device emulation and exchanging data with its slave devices. It has nothing to do with the frontend.

Is my understanding correct ?


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