How do I ensure buffer memory is aligned?

Question

I am using a hardware interface to send data that requires me to set up a DMA buffer, which needs to be aligned on 64 bits boundaries.

The DMA engine expects buffers to be aligned on at least 32 bits boundaries (4 bytes). For optimal performance the buffer should be aligned on 64 bits boundaries (8 bytes). The transfer size must be a multiple of 4 bytes.

I use posix_memalign to create a buffer like this...

posix_memalign ((void**)&pPattern, 0x1000, DmaBufferSizeinInt32s * sizeof(int) ) )

pPattern is a pointer to an int, and is the start of my buffer which is DmaBufferSizeinInt32s deep.

Is my buffer aligned on 64bits?

Answer 1

Yes, your buffer IS aligned on 64-bits. It's ALSO aligned on a 4 KByte boundary (hence the 0x1000). If you don't want the 4 KB alignement then pass 0x8 instead of 0x1000 ...

Edit: I would also note that usually when writing DMA chains you are writing them through uncached memory or through some kind of non-cache based write queue. If this is the case you want to align your DMA chains to the cache line size as well to prevent a cache write-back overwriting the start or end of your DMA chain.

Answer 2

As Goz pointed out, but (imo) a bit less clearly: you're asking for alignment by 0x1000 bytes (the second argument), which is much more than 64 bits.

You could change the call to just:

posix_memalign ((void**)&pPattern, 8, DmaBufferSizeinInt32s * sizeof(int)))

This might make the call cheaper (less wasted memory), and in any case is clearer, since you ask for something that more closely matches what you actually want.

Answer 3

I don't know your hardware and I don't know how you are getting your pPattern pointer, but this seems risky all around. Most DMA I am familiar with requires physical continuous RAM. The operating system only provides virtually continuous RAM to user programs. That means that a memory allocation of 1 MB might be composed of up to 256 unconnected 4K RAM pages.

Much of the time memory allocations will be made of continuous physical pieces which can lead to things working most of the time but not always. You need a kernel device driver to provide safe DMA.

I wonder about this because if your pPattern pointer is coming from a device driver, then why do you need to align it more?