Hello ARM experts:
In chapter K14.5.4 Ordering of Memory-mapped device control with payloads of ARMv8 reference manual(version:K.a), one of the example is:
• When a DMA peripheral has written to a buffer of data in memory, and the processing element is reading a status register to determine that the DMA transfer has completed, and then is reading the data.
It says "A DMB, or load-acquire, is not sufficient as this problem is not solely concerned with observation order, since the polling read is actually a read of a status register at a Completer, not the polling a data value that has been written by an observer."
So, for this case, the code is therefore:
P1 WAIT ([X4] == 1) ; X4 contains the address of the status register, ; and the value '1' indicates completion of the DMA transfer DSB <domain> LDR W5, [X2] ; reads data from the data buffer
But I think dmb is enough since dmb ensures no reordering.When P1 observes [4] = 1, it exits the loop and than reads data from the data buffer.
So what do I misunderstand？Thanks
Hi Alwin Joshy,
Do you mean to replace the DSB instruction in "K14.4 Using a mailbox to send an interrupt" by a DMB OSHST?
As per Will's commit message above, this is now possible, in most of the cases, but some exceptions remain:
Memory-mapped, DMA-capable peripherals that are private to a CPU (i.e. inaccessible to other CPUs) still require the DSB.
Therefore you need to consider your specific case carefully.
Yep, that's the one. Thanks for the clarification.