Thanks!
I could still read this two ways however: one where the memory is on the peripheral, and one where the memory is main memory, where the peripheral is copying to/from using DMA. Which one is it?
I could still read this two ways however: one where the memory is on the peripheral, and one where the memory is main memory, where the peripheral is copying to/from using DMA. Which one is it?
You send the device a circular list of descriptors (pointers) to a region of main memory.
In order to send data to the device, you write your network packet to the memory region associated with the pointer of the current ‘head’ of the descriptor list.
So far, you have a ring of pointers, one of those pointers points to a location you just wrote to in ram.
You then tell the device that the head of the list has changed (as you just wrote some data to the region that the head of the list is pointing to - so it can consume that pointer), the device then goes ahead and copies the data from ram into an internal buffer on the card. Once the data is consumed, the tail pointer of the ring buffer is updated to indicate that the card is finished with that memory region.
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> Typically with devices like network cards (that also operate over PCI-E) You send the device a circular list of descriptors (pointers) to a region of main memory. In order to send data to the device, you write your network packet to the memory region associated with the pointer of the current ‘head’ of the descriptor list. So far, you have a ring of pointers, one of those pointers points to a location you just wrote to in ram. You then tell the device that the head of the list has changed (as you just wrote some data to the region that the head of the list is pointing to - so it can consume that pointer), the device then goes ahead and copies the data from ram into an internal buffer on the card. Once the data is consumed, the tail pointer of the ring buffer is updated to indicate that the card is finished with that memory region.
But equally a peripheral can expose its own memory and ask the host to write into it.
Cheap devices tend to do the former because it avoids the need to have expensive memory built in. They can just “borrow” system memory. More expensive, performance optimised, devices tend to do the latter.
It’s also worth mentioning that DMA tends to work between every device attached to the PCIe bus. So Microsoft’s DirectStorage API seems to be using this feature, by having the GPU directly read data from an SSD, without the data ever touching the CPU or main memory.