i2c: virtio: Add support for zero-length requests

The virtio specification received a new mandatory feature
(VIRTIO_I2C_F_ZERO_LENGTH_REQUEST) for zero length requests. Fail if the
feature isn't offered by the device.

For each read-request, set the VIRTIO_I2C_FLAGS_M_RD flag, as required
by the VIRTIO_I2C_F_ZERO_LENGTH_REQUEST feature.

This allows us to support zero length requests, like SMBUS Quick, where
the buffer need not be sent anymore.

Signed-off-by: Viresh Kumar <[e-mail]>
Link: https://lore.kernel.org/r/7c58[e-mail]
Signed-off-by: Michael S. Tsirkin <[e-mail]>
Acked-by: Jie Deng <[e-mail]> # once the spec is merged

drivers/i2c/busses/i2c-virtio.c

···
 		int outcnt = 0, incnt = 0;
 
 		/*
-		 * We don't support 0 length messages and so filter out
-		 * 0 length transfers by using i2c_adapter_quirks.
-		 */
-		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 (msgs[i].flags & I2C_M_RD)
+			reqs[i].out_hdr.flags |= cpu_to_le32(VIRTIO_I2C_FLAGS_M_RD);
+
 		if (i != num - 1)
-			reqs[i].out_hdr.flags = cpu_to_le32(VIRTIO_I2C_FLAGS_FAIL_NEXT);
+			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;
+		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);
+			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;
+			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;
···
 
 static u32 virtio_i2c_func(struct i2c_adapter *adap)
 {
-	return I2C_FUNC_I2C | (I2C_FUNC_SMBUS_EMUL & ~I2C_FUNC_SMBUS_QUICK);
+	return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL;
 }
 
 static struct i2c_algorithm virtio_algorithm = {
···
 	.functionality = virtio_i2c_func,
 };
 
-static const struct i2c_adapter_quirks virtio_i2c_quirks = {
-	.flags = I2C_AQ_NO_ZERO_LEN,
-};
-
 static int virtio_i2c_probe(struct virtio_device *vdev)
 {
 	struct virtio_i2c *vi;
 	int ret;
+
+	if (!virtio_has_feature(vdev, VIRTIO_I2C_F_ZERO_LENGTH_REQUEST)) {
+		dev_err(&vdev->dev, "Zero-length request feature is mandatory\n");
+		return -EINVAL;
+	}
 
 	vi = devm_kzalloc(&vdev->dev, sizeof(*vi), GFP_KERNEL);
 	if (!vi)
···
 	snprintf(vi->adap.name, sizeof(vi->adap.name),
 		 "i2c_virtio at virtio bus %d", vdev->index);
 	vi->adap.algo = &virtio_algorithm;
-	vi->adap.quirks = &virtio_i2c_quirks;
 	vi->adap.dev.parent = &vdev->dev;
 	vi->adap.dev.of_node = vdev->dev.of_node;
 	i2c_set_adapdata(&vi->adap, vi);
···
 }
 #endif
 
+static const unsigned int features[] = {
+	VIRTIO_I2C_F_ZERO_LENGTH_REQUEST,
+};
+
 static struct virtio_driver virtio_i2c_driver = {
-	.id_table	= id_table,
-	.probe		= virtio_i2c_probe,
-	.remove		= virtio_i2c_remove,
-	.driver	= {
+	.feature_table		= features,
+	.feature_table_size	= ARRAY_SIZE(features),
+	.id_table		= id_table,
+	.probe			= virtio_i2c_probe,
+	.remove			= virtio_i2c_remove,
+	.driver			= {
 		.name	= "i2c_virtio",
 	},
 #ifdef CONFIG_PM_SLEEP

include/uapi/linux/virtio_i2c.h

···
 #include <linux/const.h>
 #include <linux/types.h>
 
+/* Virtio I2C Feature bits */
+#define VIRTIO_I2C_F_ZERO_LENGTH_REQUEST	0
+
 /* The bit 0 of the @virtio_i2c_out_hdr.@flags, used to group the requests */
 #define VIRTIO_I2C_FLAGS_FAIL_NEXT	_BITUL(0)
+
+/* The bit 1 of the @virtio_i2c_out_hdr.@flags, used to mark a buffer as read */
+#define VIRTIO_I2C_FLAGS_M_RD		_BITUL(1)
 
 /**
  * struct virtio_i2c_out_hdr - the virtio I2C message OUT header