caif_virtio: Introduce caif over virtio

Add the CAIF Virtio shared memory driver for talking
to a modem.

This CAIF Link layer communicates to the modem over
shared memory. It is implemented as a virtio_driver.
The underlying virtio device is managed by the remoteproc
framework. The Virtio queue is used for transmitting data
to the modem, and the new vringh is used for receiving data.

Genalloc is used for managing the shared memory used for TX
data. The default dma-alloc-coherent allocator can only
allocate whole pages, and this wastes too much shared memory.

Flow control is implemented by stopping the TX-queues if the
virtio queues go full or we run out of memory. Queued are
reopened when queues are below the watermark.

NAPI is used in RX path, and a dedicated tasklet is used
for releasing TX buffers.

Signed-off-by: Erwan Yvin <erwan.yvin@stericsson.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (minor fixes)

drivers/net/caif/Kconfig

@@ -47,3 +47,17 @@ config CAIF_HSI
        The caif low level driver for CAIF over HSI.
        Be aware that if you enable this then you also need to
        enable a low-level HSI driver.
+
+config CAIF_VIRTIO
+	tristate "CAIF virtio transport driver"
+	depends on CAIF
+	select VHOST_RING
+	select VIRTIO
+	select GENERIC_ALLOCATOR
+	default n
+	---help---
+	The caif driver for CAIF over Virtio.
+
+if CAIF_VIRTIO
+source "drivers/vhost/Kconfig"
+endif

drivers/net/caif/Makefile

@@ -13,3 +13,6 @@ obj-$(CONFIG_CAIF_SHM) += caif_shm.o
 
 # HSI interface
 obj-$(CONFIG_CAIF_HSI) += caif_hsi.o
+
+# Virtio interface
+obj-$(CONFIG_CAIF_VIRTIO) += caif_virtio.o