[Question about DMA] Consistent memory?

[Masahiro Yamada]

Hi Alan, Mike,

Thanks for your help!


2015-12-31 19:25 GMT+09:00 One Thousand Gnomes <gnomes at lxorguk.ukuu.org.uk>:

>>
>> In a system like Fig.2, is the memory non-consistent?
>
> dma_alloc_coherent will always provide you with coherent memory. On a
> machine with good cache interfaces it will provide you with normal
> memory. On some systems it may be memory from a special window, in other
> cases it will fall back to providing uncached memory for this.
>
> If the platform genuinely cannot support this (even by marking those areas
> uncacheable) then it will fail the allocation.
>
> What it does mean is that you need to use non-coherent mappings when
> accessing a lot of data. On hardware without proper cache coherency it
> may be quite expensive to access coherent memory.


Now, it is clearer to me.
The following is what I understood.
(Please point out if I am wrong.)


I think, roughly, there are two ways for handling DMA:
(At first, I was so confused that I was thinking about [1] and [2] mixed.)



[1] DMA-coherent buffers

Allocate buffers with dma_alloc_coherent()
and just have access to the buffers without cache synchronization.

There is no need to call dma_sync_single_for_*().



[2] Streaming DMA

Allocate buffers with kmalloc() or friends,
and then map them for DMA with dma_map_single().

The buffers are cached, so they are non-consitent
unless there exists hardware assist such as
Cache Coherency Interconnect.

The drivers must invoke cache operations
by calling dma_sync_single_for_*().




Is there any guideline about which way should be used in drivers?

I think, if the buffer size is small, [1] is more efficient
because it need not invoke cache operations.

If the buffer is large, [2] seems better because
the cost of uncached memory access gets more expensive
than that of cache operations.

(If devices are connected to the memory controller
via Cache Coherency Interconnect, [1] always works very well.
But drivers should be written in a portable way, so
such a hardware implementation should not be expected.)

I am not sure about the border line between [1] and [2], though...



BTW, I am studying the DMA APIs in order to write a new
MMC host driver for my ARM SoC.


I grepped under drivers/mmc/host, and
I found many drivers call dma_alloc_coherent(),
but there are also some drivers that use dma_map_single().




-- 
Best Regards
Masahiro Yamada