This patch-set introduces the CAIF Virtio Link layer. The purpose is to communicate with a remote processor (a modem) over shared memory. Virtio is used as the transport mechanism, and the Remoteproc framework provides configuration and management of the Virtio rings and devices. The modem and Linux host may be on the same SoC, or connected over a shared memory interface such as LLI.
Zero-Copy data transport on the modem is primary goal for CAIF Virtio. In order to achieve Zero-Copy the direction of the Virtio rings are flipped in the RX direction. So we have implemented the Virtio access-function similar to what is found in vhost.c. The connected LTE-modem is a multi-processor system with an advanced memory allocator on board. In order to provide zero-copy from the modem to the connected Linux host, the direction of the Virtio rings are reversed. This allows the modem to allocate data-buffers in RX direction and pass them to the Linux host, and recycled buffers to be sent back to the modem. The option of providing pre-allocated buffers in RX direction has been considered, but rejected. The allocation of data buffers happens deep down in the network signaling stack on the LTE modem before it is known what type of data is received. It may be data that is handled within the modem and never sent to the Linux host, or IP traffic going to the host. Pre-allocated Virtio buffers does not fit for this usage. Another issue is that the network traffic pattern may vary, resulting in variation of number and size of buffers allocated from the memory allocator. Dynamic allocation is needed in order to utilize memory properly. Due to this, we decided we had to implement "reversed" vrings. Reversed vrings allows us to minimize the impact on the current memory allocator and buffer handling on the modem. In order to implement reversed rings we have added functions for reading descriptors from the available-ring and adding descriptors to the used-ring. The internal data-structures in virtio_ring.c are moved into a new header file so the data-structures can be accessed by caif_virtio. The data buffers in TX direction are allocated using dma_alloc_coherent(). This allows memory to be allocated from the memory region shared between the Host and modem. In TX direction single linearized TX buffers are added to the vring. In RX direction linearized frames are also used, but multiple descriptors may be linked. This is done to allow maximum efficiency for the LTE modem. This patch set is not yet fully tested and does not handle all negative scenarios correctly. So at this stage we're primarily looking for review comments related to the structure of the Virtio code. There are several options on how to structure this, and feedback is welcomed. Thanks, Sjur Sjur Brændeland (4): virtio: Move definitions to header file vring.h include/vring.h: Add support for reversed vritio rings. virtio_ring: Call callback function even when used ring is empty caif_virtio: Add CAIF over virtio drivers/net/caif/Kconfig | 9 + drivers/net/caif/Makefile | 3 + drivers/net/caif/caif_virtio.c | 627 ++++++++++++++++++++++++++++++++ drivers/remoteproc/remoteproc_virtio.c | 2 +- drivers/virtio/virtio_ring.c | 102 +----- drivers/virtio/vring.h | 124 +++++++ include/linux/virtio_ring.h | 8 +- include/uapi/linux/virtio_ids.h | 1 + 8 files changed, 776 insertions(+), 100 deletions(-) create mode 100644 drivers/net/caif/caif_virtio.c create mode 100644 drivers/virtio/vring.h -- 1.7.9.5 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
