Can I call dma_map_single() on DeviceB using an addresses returned from dma_alloc_coherent on DeviceA?

Question

I am writing custom linux driver that needs to DMA memory around between multiple PCIE devices. I have the following situation:

1. I'm using dma_alloc_coherent to allocate memory for DeviceA
2. I then use DeviceA to fill the memory buffer.

Everything is fine so far but at this point I would like to DMA the memory to DeviceB and I'm not sure the proper way of doing it.

For now I am calling dma_map_single for DeviceB using the address returned from dma_alloc_coherent called on DeviceA. This seems to work fine in x86_64 but it feels like I'm breaking the rules because:

1. dma_map_single is supposed to be called with memory allocated from kmalloc ("and friends"). Is it problem being called with an address returned from another device's dma_alloc_coherent call?

2. If #1 is "ok", then I'm still not sure if it is necessary to call the dma_sync_* functions which are needed for dma_map_single memory. Since the memory was originally allocated from dma_alloc_coherent, it should be uncached memory so I believe the answer is "dma_sync_* calls are not necessary", but I am not sure.

I'm worried that I'm just getting lucky having this work and a future kernel update will break me since it is unclear if I'm following the API rules correctly. My code eventually will have to run on ARM and PPC too, so I need to make sure I'm doing things in a platform independent manner instead of getting by with some x86_64 architecture hack.

I'm using this as a reference: https://www.kernel.org/doc/html/latest/core-api/dma-api.html

Answer 1

1. dma_alloc_coherent() acts similarly to __get_free_pages() but as size granularity rather page, so no issue I would guess here.
2. First call dma_mapping_error() after dma_map_single() for any platform specific issue. dma_sync_*() helpers are used by streaming DMA operation to keep device and CPU in sync. At minimum dma_sync_single_for_cpu() is required as device modified buffers access state need to be sync before CPU use it.