[RFC PATCH] ARM: Change the mandatory barriers implementation

[Abhijeet Dharmapurikar]

Catalin Marinas wrote:
> On Thu, 2010-02-04 at 13:15 +0000, Catalin Marinas wrote:
>> On Thu, 2010-02-04 at 00:21 +0000, Abhijeet Dharmapurikar wrote:
>>>> The mandatory barriers (mb, rmb, wmb) are used even on uniprocessor
>>>> systems for things like ordering Normal Non-cacheable memory accesses
>>>> with DMA transfer (via Device memory writes). The current implementation
>>>> uses dmb() for mb() and friends but this is not sufficient. The DMB only
>>>> ensures the ordering of accesses with regards to a single observer
>>>> accessing the same memory. If a DMA transfer is started by a write to
>>>> Device memory, the data to be transfered may not reach the main memory
>>>> (even if mapped as Normal Non-cacheable) before the device receives the
>>>> notification to begin the transfer. The only barrier that would help in
>>>> this situation is DSB which would completely drain the write buffers.
>>> On ARMv7, DMB guarantees that all accesses prior to DMB are observed by
>>> an observer if that observer sees any accesses _after_ the DMB. In this
>>> case, since DMA engine observes a write to itself( It is being written
>>> to and hence must observe the write) it should also see the writes to
>>> the buffers. A dmb() after the writes to buffer and before write to DMA
>>> engine should suffice.
>> I asked our processor architect for a clarification on the wording of
>> the DMB definition but the "all accesses" part most likely refer to
>> accesses to the same peripheral or memory block (but not together).
> 
> I got some clarification and there is nothing wrong with the definition
> of the DMB. The catch here is that "observe" (as per the ARM ARM) is
> defined only for master accesses. The DMA engine in this case above does
> not "observe" the write to itself as this is a slave access to one of
> its memory-mapped ports.
> 
> So, the code below:
> 
> 	STR	[Normal non-cacheable]
> 	DMB
> 	STR	[Device]
> 
> puts the first store in Group A (according to the DMB definition) and
> the second store in Group B but since the DMA device does not "observe"
> Group B in this case, there is no requirement for the ordering between
> the observability of the store to normal memory and the observability of
> the side-effects of the store to the DMA device.

I agree now. DSB would be the right thing to do. Moreover the option to 
have a barrier.h for each platform gives them a chance to fine tune 
these barriers further.