CODEWITHSUNDEEP
cpu-architecturedma
mclaassen
mclaassen

Reputation: 5128

How/does DMA handle multiple concurrent transfers?

I am working on implementing a VM and trying to model all the different hardware components as accurately as possible, just for pure learning purposes.

My question is, how does a DMA device handle multiple concurrent transfer requests? From what I understand a DMA device has several registers to set the location in memory, the type of operation (read or write) and the number of bytes, so what happens when the CPU requests an operation from DMA, puts the thread to sleep and then the next thread that runs also requests a DMA operation while the previous one is still in progress? Is this even supported?

Upvotes: 2

Views: 3052

Answers (1)

Dolda2000
Dolda2000

Reputation: 25855

Unless you're talking about ancient, ISA-era hardware, DMA nowadays is handled by the device itself taking ownership of the bus and requesting the data directly from the RAM. See the Wikipedia article on Bus Mastering for more information.

Therefore, it is really up to any individual device how to handle DMA, so not much can be said for the general case. However, most simple devices just support a single DMA operation at a time; if the host wants to submit two DMA operations, it would simply wait for the first DMA to complete (being notified by an interrupt) and then instruct the device to do the second one, the OS putting the requesting thread to sleep while the first DMA is in progress. There are certainly variations, however, such as using a command-buffer that can specify multiple (DMA-involving or not) operations for the device to do in sequence without interrupting the CPU between each.

I doubt there are very many devices at all that try to carry out multiple transfers simultaneously, however, seeing as how interleaving DRAM accesses would just hurt performance anyway. But I wouldn't exclude their existence, especially if the operations involve very large transfers.

In the end, you'll just have to read up on the particular device you're trying to emulate.

Upvotes: 3

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