DMA setup critical region

[pini]

It is commonly admitted that the DMA setup (I mean here setting up page, address and count DMA registers for a transfer to come) has to be embedded between

Code: Select all

cli

and

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sti

, so that it cannot be interrupted.

My question is: is this necessary ? what is the risk when being interrupted ?

Thanks in advance

pini

[Brendan]

Hi,

As far as I can tell, as long as an IRQ (or another thread if an IRQ can cause a thread switch) can't mess with the same DMA channel that you're setting up you may not need to disable interrupts.

There are probably some unusual circumstances (e.g. where a device driver can setup a DMA transfer outside of it's IRQ handler and/or within it's IRQ handler) that could break things. One example would be an IRQ handler that could read a DMA channel's count while it's in the middle of being set.

I guess disabling interrupts is fast and it's good insurance (just to make sure). It's certainly easier than trying to think of all of the problems that could occur, or writing a list of unusual situations that device driver writers need to be careful of.


Cheers,

Brendan