Back in December 2011, LWN reviewed the list of Android kernel patches in the linux-next staging directory. The merging of these drivers, one of which is a memory allocator called PMEM, holds the promise that the mainline kernel release can one day boot an Android user space. Since then, it has become clear that PMEM is considered obsolete and will be replaced by the ION memory manager. ION is a generalized memory manager that Google introduced in the Android 4.0 ICS (Ice Cream Sandwich) release to address the issue of fragmented memory management interfaces across different Android devices. There are at least three, probably more, PMEM-like interfaces. On Android devices using NVIDIA Tegra, there is "NVMAP"; on Android devices using TI OMAP, there is "CMEM"; and on Android devices using Qualcomm MSM, there is "PMEM" . All three SoC vendors are in the process of switching to ION.

This article takes a look at ION, summarizing its interfaces to user space and to kernel-space drivers. Besides being a memory pool manager, ION also enables its clients to share buffers, hence it treads the same ground as the DMA buffer sharing framework from Linaro (DMABUF). This article will end with a comparison of the two buffer sharing schemes.

ION heaps

Like its PMEM-like predecessors, ION manages one or more memory pools, some of which are set aside at boot time to combat fragmentation or to serve special hardware needs. GPUs, display controllers, and cameras are some of the hardware blocks that may have special memory requirements. ION presents its memory pools as ION heaps. Each type of Android device can be provisioned with a different set of ION heaps according to the memory requirements of the device. The provider of an ION heap must implement the following set of callbacks:

struct ion_heap_ops {
	int (*allocate) (struct ion_heap *heap,
			 struct ion_buffer *buffer, unsigned long len,
			 unsigned long align, unsigned long flags);
	void (*free) (struct ion_buffer *buffer);
	int (*phys) (struct ion_heap *heap, struct ion_buffer *buffer,
		     ion_phys_addr_t *addr, size_t *len);
	struct scatterlist *(*map_dma) (struct ion_heap *heap,
			 struct ion_buffer *buffer);
	void (*unmap_dma) (struct ion_heap *heap, 
	         struct ion_buffer *buffer);
	void * (*map_kernel) (struct ion_heap *heap, 
	         struct ion_buffer *buffer);
	void (*unmap_kernel) (struct ion_heap *heap, 
	         struct ion_buffer *buffer);
	int (*map_user) (struct ion_heap *heap, struct ion_buffer *buffer,
			 struct vm_area_struct *vma);
   };

Briefly, allocate() and free() obtain or release an ion_buffer object from the heap. A call to phys() will return the physical address and length of the buffer, but only for physically-contiguous buffers. If the heap does not provide physically contiguous buffers, it does not have to provide this callback. Here ion_phys_addr_t is a typedef of unsigned long, and will, someday, be replaced by phys_addr_t in include/linux/types.h. The map_dma() and unmap_dma() callbacks cause the buffer to be prepared (or unprepared) for DMA. The map_kernel() and unmap_kernel() callbacks map (or unmap) the physical memory into the kernel virtual address space. A call to map_user() will map the memory to user space. There is no unmap_user() because the mapping is represented as a file descriptor in user space. The closing of that file descriptor will cause the memory to be unmapped from the calling process.

The default ION driver (which can be cloned from here) offers three heaps as listed below:

   ION_HEAP_TYPE_SYSTEM:        memory allocated via vmalloc_user().
   ION_HEAP_TYPE_SYSTEM_CONTIG: memory allocated via kzalloc.
   ION_HEAP_TYPE_CARVEOUT:	carveout memory is physically contiguous and set aside at boot.

Developers may choose to add more ION heaps. For example, this NVIDIA patch was submitted to add ION_HEAP_TYPE_IOMMU for hardware blocks equipped with an IOMMU.

Using ION from user space

Typically, user space device access libraries will use ION to allocate large contiguous media buffers. For example, the still camera library may allocate a capture buffer to be used by the camera device. Once the buffer is fully populated with video data, the library can pass the buffer to the kernel to be processed by a JPEG encoder hardware block.

A user space C/C++ program must have been granted access to the /dev/ion device before it can allocate memory from ION. A call to open("/dev/ion", O_RDONLY) returns a file descriptor as a handle representing an ION client. Yes, one can allocate writable memory with an O_RDONLY open. There can be no more than one client per user process. To allocate a buffer, the client needs to fill in all the fields except the handle field in this data structure:

struct ion_allocation_data {
        size_t len;
        size_t align;
        unsigned int flags;
        struct ion_handle *handle;
   }

The handle field is the output parameter, while the first three fields specify the alignment, length and flags as input parameters. The flags field is a bit mask indicating one or more ION heaps to allocate from, with the fallback ordered according to which ION heap was first added via calls to ion_device_add_heap() during boot. In the default implementation,ION_HEAP_TYPE_CARVEOUT is added before ION_HEAP_TYPE_CONTIG. The flags of ION_HEAP_TYPE_CONTIG | ION_HEAP_TYPE_CARVEOUT indicate the intention to allocate from ION_HEAP_TYPE_CARVEOUT with fallback to ION_HEAP_TYPE_CONTIG.

User-space clients interact with ION using the ioctl() system call interface. To allocate a buffer, the client makes this call:

int ioctl(int client_fd, ION_IOC_ALLOC, struct ion_allocation_data *allocation_data)

This call returns a buffer represented by ion_handle which is not a CPU-accessible buffer pointer. The handle can only be used to obtain a file descriptor for buffer sharing as follows:

int ioctl(int client_fd, ION_IOC_SHARE, struct ion_fd_data *fd_data);

Here client_fd is the file descriptor corresponding to /dev/ion, and fd_data is a data structure with an input handle field and an output fd field, as defined below:

struct ion_fd_data {
        struct ion_handle *handle;
        int fd;
   }

The fd field is the file descriptor that can be passed around for sharing. On Android devices the BINDERIPC mechanism may be used to send fd to another process for sharing. To obtain the shared buffer, the second user process must obtain a client handle first via the open("/dev/ion", O_RDONLY) system call. ION tracks its user space clients by the PID of the process (specifically, the PID of the thread that is the "group leader" in the process). Repeating the open("/dev/ion", O_RDONLY) call in the same process will get back another file descriptor corresponding to the same client structure in the kernel.

To free the buffer, the second client needs to undo the effect of mmap() with a call to munmap(), and the first client needs to close the file descriptor it obtained via ION_IOC_SHARE, and call ION_IOC_FREE as follows:

int ioctl(int client_fd, ION_IOC_FREE, struct ion_handle_data *handle_data);

Here ion_handle_data holds the handle as shown below:

     struct ion_handle_data {
	     struct ion_handle *handle;
     }

The ION_IOC_FREE command causes the handle‘s reference counter to be decremented by one. When this reference counter reaches zero, the ion_handle object gets destroyed and the affected ION bookkeeping data structure is updated.

User processes can also share ION buffers with a kernel driver, as explained in the next section.

Sharing ION buffers in the kernel

In the kernel, ION supports multiple clients, one for each driver that uses the ION functionality. A kernel driver calls the following function to obtain an ION client handle:

struct ion_client *ion_client_create(struct ion_device *dev, 
                   unsigned int heap_mask, const char *debug_name)

The first argument,dev, is the global ION device associated with /dev/ion; why a global device is needed, and why it must be passed as a parameter, is not entirely clear. The second argument,heap_mask, selects one or more ION heaps in the same way as the ion_allocation_data. The flags field was covered in the previous section. For smart phone use cases involving multimedia middleware, the user process typically allocates the buffer from ION, obtains a file descriptor using the ION_IOC_SHARE command, then passes the file desciptor to a kernel driver. The kernel driver calls ion_import_fd() which converts the file descriptor to an ion_handle object, as shown below:

struct ion_handle *ion_import_fd(struct ion_client *client, int fd_from_user);

The ion_handle object is the driver‘s client-local reference to the shared buffer. The ion_import_fd() call looks up the physical address of the buffer to see whether the client has obtained a handle to the same buffer before, and if it has, this call simply increments the reference counter of the existing handle.

Some hardware blocks can only operate on physically-contiguous buffers with physical addresses, so affected drivers need to convertion_handle to a physical buffer via this call:

int ion_phys(struct ion_client *client, struct ion_handle *handle,
	       ion_phys_addr_t *addr, size_t *len)

Needless to say, if the buffer is not physically contiguous, this call will fail.

When handling calls from a client, ION always validates the input file descriptor, client and handle arguments. For example, when importing a file descriptor, ION ensures the file descriptor was indeed created by an ION_IOC_SHARE command. When ion_phys() is called, ION validates whether the buffer handle belongs to the list of handles the client is allowed to access, and returns error if the handle is not on the list. This validation mechanism reduces the likelihood of unwanted accesses and inadvertent resource leaks.

ION provides debug visibility through debugfs. It organizes debug information under /sys/kernel/debug/ion, with bookkeeping information in stored files associated with heaps and clients identified by symbolic names or PIDs.

Comparing ION and DMABUF

ION and DMABUF share some common concepts. The dma_buf concept is similar to ion_buffer, while dma_buf_attachment serves a similar purpose as ion_handle. Both ION and DMABUF use anonymous file descriptors as the objects that can be passed around to provide reference-counted access to shared buffers. On the other hand, ION focuses on allocating and freeing memory from provisioned memory pools in a manner that can be shared and tracked, while DMABUF focuses more on buffer importing, exporting and synchronization in a manner that is consistent with buffer sharing solutions on non-ARM architectures.

The following table presents a feature comparison between ION and DMABUF:

ION and DMABUF can be separately integrated into multimedia applications written using the Video4Linux2 API. In the case of ION, these multimedia programs tend to use PMEM now on Android devices, so switching to ION from PMEM should have a relatively small impact.

Integrating DMABUF into Video4Linux2 is another story. It has taken ten patches to integrate the videobuf2 mechanism with DMABUF; in fairness, many of these revisions were the result of changes to DMABUF as that interface stabilized. The effort should pay dividends in the long run because the DMABUF-based sharing mechanism is designed with DMA mapping hooks for CMA and IOMMU. CMA and IOMMU hold the promise to reduce the amount of carve out memory that it takes to build an Android smart phone. In this email, Andrew Morton was urging the completion of the patch review process so that CMA can get through the 3.4 merge window.

Even though ION and DMABUF serve similar purposes, the two are not mutually exclusive. The Linaro Unified Memory Management team has started to integrate CMA into ION. To reach the state where a release of the mainline kernel can boot the Android user space, the /dev/ion interface to user space must obviously be preserved. In the kernel though, ION drivers may be able to use some of the DMABUF APIs to hook into CMA and IOMMU to take advantage of the capabilities offered by those subsystems. Conversely, DMABUF might be able to leverage ION to present a unified interface to user space, especially to the Android user space. DMABUF may also benefit from adopting some of the ION heap debugging features in order to become more developer friendly. Thus far, many signs indicate that Linaro, Google, and the kernel community are working together to bring the combined strength of ION and DMABUF to the mainline kernel.

The Android ION memory allocator