Allocating a physical memory buffer in linux

1
votes

I have an SoC which has both DSP and ARM cores on it and I would like to create a section of shared memory that both my userspace software, and DSP software are able to access. What would be the best way to allocate a buffer like this in Linux? Here is a little background, right now what I have is a kernel module in which I use kmalloc() to get a kernel buffer, I then use the __pa() macro from asm/page.h to get the physical address of my kernel buffer. I save this address as a sysfs entry so that my userspace code can get the physical address of this buffer. I can then write this address to the DSP so it knows where the shared memory location is, and I can also mmap /dev/mem or my own kernel module so that I can access this buffer from userspace (I could also use the read/write fileops).

For some reason I feel like this is overboard but I cannot find the best way to do what I am trying to do.

Would it be possible to just mmap \dev\mem a section of memory and just read and write to this section? My feeling is that this would not 'lock' this section of memory from the kernel, thus the kernel could still read/write to this memory without me knowing. Is this the case. After reading the memory management chapter of LDD3 I see that mmap creates a new VMA of the mapping. Would this lock this area of memory so that other processes would not get allocated this section of memory?

Any and all help is appreciated

2
memory-managementlinux-kernel
Are the cores both on the same physical memory bus? On Android, for example, a typical shared memory architecture uses /dev/pmem and there are system components to transfer data between the cores using an RPC mechanism. – Peter L.

2 Answers

2
votes

Depending on the kind of DMA you're using, you need to allocate the buffer with dma_alloc_coherent(), or use standard allocations and the dma_map_* functions. (You must not use __pa(); physical addresses are not necessarily the same as DMA bus addresses.)

To map the buffers to user space, use dma_mmap_coherent() for coherent buffers, or map the memory pages manually for streaming buffers.

0
votes

For a similar requirement of mine, I had reserved about 16 MB of memory towards the end of ram and used it in both kernel and user space. Suppose you have 128 MB ram, you can set BOOTMEM argument as 112 MB in your boot loader. I am assuming you are using uboot. This will reserve 16 MB towards the end of the ram. Now in kernel and user space you can map this area and use it as shared memory.